I've edited this post, after having done more research. I've narrowed down the currently available and affordable GPS computers that have maps and an acceptable battery life, to 3 models: Bryton Rider 450, Lezyne Mega series, and Wahoo Bolt. The Bryton and Lezyne have very long battery life, but they're both missing some functions that the Wahoo has. Sadly, the Wahoo has much shorter battery life. There doesn't seem to be one model that has it all.

Bryton Rider 450
Advantage: long battery life (35 hours). Can open routes from internal memory. Has Galileo and other alternative GPS systems for better accuracy and wider coverage.
Disadvantage: Cannot import GPX through smartphone, GPX files have to be transferred from PC through a USB cable. Not compatible with TCX and cue sheets. No sync with third party services like Komoot or RWGPS.(except Strava). Lack of communication with customer service.

Lezyne Mega Series
Advantage: long battery life (32 or 48 hours). Can import GPX and TCX from Lezyne Root website and then sync to app. Better spontaneous route creation in Lezyne Ally app than Bryton Active app.
Disadvantage: Cannot open routes from internal memory, so if powered off and then turned on, device must connect with smartphone again to reload routes. No sync with Komoot or RWGPS.

Wahoo Bolt
Advantage: Can open routes from internal memory. Can import GPX and TCX from app. Sync with many third party services. Better spontaneous route creation in Wahoo Element app than Bryton Active app.
Disadvantage: short battery life (15 hours). Most expensive.


Battery life
For multi day trips with limited charging, I think devices need at least 24-30 hours of charge while running GPS and 2 sensors with an active display. The Wahoo Bolt's 15 hours is not enough.

Smartphone dependency
All models require a smartphone for transferring routes to the device. After the routes are stored in local memory, the Bryton and Wahoo devices can open any of the stored routes at any time in the future. So you don't need to reconnect the phone again. But the Lezyne Mega requires a phone connection every time you want to reopen or change to another route, especially if you turn off the device and then turn it back on. This makes the Lezyne completely dependent on a phone when navigating. If you go on a long trip, it would be better to have routes saved on 2 independent devices as a backup (GPS computer and smartphone) in case one device fails or runs out of battery power. This could also be useful on hiking trips. But if you are only using the devices for short daily commutes, perhaps it's not such a big issue.

Route Creation
All 3 devices have companion phone apps that can generate routes. Bryton and Lezyne also have websites that can create routes, but are inferior in quality to the apps, so you might as well do it in the phone. Both Bryton and Lezyne also can review your recorded tracks on the website. Wahoo does not have a website interface, although this is not a problem because you can sync your tracks to other sites with better analysis. The Bryton app only gives you 1 path option during route creation, usually the shortest path, which might not be the best route for bicycles. If you don't like the path, you'll have to insert additional waypoints to try and manipulate the output. The Lezyne and Wahoo apps both give alternative options between waypoints, which can help create a better route, but I don't know if the output is any more bicycle friendly. But I think Komoot and RWGPS create much better routes, which leads to "third party support".

Third Party Support
Wahoo has the best third party support, with the ability to sync with several alternate route creation services. So if you're on the fly and need a quick route, but the device companion app isn't giving you a good one, you can switch to the Komoot app to create a better route, save it, and then quickly sync it to the Wahoo App for transfer to the GPS device. If the third party service is not supported, you can import the GPX or TCX directly with the Wahoo app. With Lezyne, you'll have to manually upload a GPX or TCX to the Lezyne website to then sync to the Lezyne app. With Bryton, you have to transfer the GPX to the device with a USB cable from a PC, which makes third party route creation unusable if you are outside on your bike (although theoretically it may be possible to transfer with OTG cable from a smartphone). Bryton can't read TCX. If you are at home planning a trip in front of a PC, not having third party support may be ok, but is still a little inconvenient.

Navigation
Alll 3 devices give you navigation. But the quality will depend on the route creation and third party support, as described above. All 3 give turn-by-turn navigation if you created the route with their companion app. If you want this with third party routes, they need to be imported with cue sheets in TCX format. Which means Bryton doesn't support third party cue sheets since it can't read TCX. I'm not sure which devices support street names in cue sheets, but some of them do.

Ride Tracking
All 3 devices can track rides. The Lezyne can do this without needing a phone connection, since tracking is a different function from navigation. All 3 support both Bluetooth and ANT+ devices, and have a built in barometer. Both Lezyne and Wahoo have electronic compasses, but I couldn't find it in the documentation for the Bryton 450, although some of Bryton's other devices have it. Ideally, you should be able to track and navigate at the same time, with the option of changing your navigation route while still tracking the same ride. I remember reading that some of the devices should be able to do this, but I don't know if all of them do. The Bryton 450 can also show charts and graphs on it's display (don't know if in real-time or after a track ends). I don't know if the Wahoo or Lezyne devices can display them. I think all 3 devices store in FIT format. One problem with the Bryton is that if you want to export to a third party, you'll have to transfer the files manually from internal memory to your PC with USB cable.

Conclusion
The Wahoo Bolt's third party support for navigation makes it a winner.... except for it's short battery life. Support for syncing with third party navigation is mainly important for short, spontaneous routes while commuting, for example riding in a city. But the short battery life makes it unsuitable for long trips without battery charging. For short rides when battery life is not an issue, I might as well use the Komoot app with its better path findng and mount my smartphone to the handlebar. Komoot doesn't record speed and cadence sensors, but it may not be necessary for short commutes. If I want to see the speed and cadence, I could use a 45€ Bryton Rider 10 (16 hours, Bluetooth) or 73€ Bryton Rider 310 (36 hours, ANT+). With an ANT+ computer, you have the option to connect sensors both to the computer and your smartphone at the same time, if the sensors have dual output of both Bluetooth and ANT+.

The Lezyne Mega series may seem like the next best option if battery life is most important.... except for the requirement for it to always have a phone connection to start navigating any route, since powering off the Lezyne device will make it lose its route.

The Bryton Rider 450 seems to be the most independent device, in every aspect, in terms of being able to operate independently from a smartphone, but also independently without third party support.


Smartphone
So then why not simply use only a smartphone, and connect it with a large capacity powerbank? I also looked at several navigation apps, but none of them have all the features to completely replace a GPS bike computer in all situations.

Komoot App
The website and app both seemed to create the best routes, for both cycling and hiking. Inputting a different type of riding or fitness level produces different paths, which seemed to help control whether it became a faster direct route or a more leisurely, interesting, or strenuous route. But the Komoot App does not connect with sensors. It also cannot use the internal barometer of a smartphone to calculate altitude, and relies solely on GPS for altitude. This would be fine if you only wanted to navigate short commutes when tracking is not necessary. But it's not really a good solution for longer trips when you want to collect biometric data. Although you could simply use a GPS computer without navigation features at the same time to run your sensors. A problem with the Komoot app is not having multiple map source options. But you can download its maps for your routes for offline navigation. Komoot app also features points of interests along the route.

Ride With GPS App
RWGPS website also creates excellent routes (although less so for hiking), but in my short tests, Komoot seemed to create better paths. RWGPS can only create routes on the website, and not in the app, which is only used for navigation and tracking. Although the app has the advantage of being able to connect with sensors for tracking. But the website also has the problem of not being optimized for small screens and touch input. So you would have to pre-plan all trips with RWGPS on a PC before being able to navigate with the app. The RWGPS app can also download offline maps. The RWGPS website also has special (paid) tools that allow you to easily split or merge routes.

Locus and OruxMaps
Locus and OruxMaps are incredibly feature rich apps that allow you to load your own custom maps and routes for navigation, mainly for the purpose of running offline maps, while tracking at the same time. OruxMaps has some of the best customization and flexibility, except that it's difficult to find the offline maps for it. I had a brief test of Locus, and it seems to have similar features with OruxMaps, but the benefit of Locus is its availability of multiple sources of offline maps for download directly from inside the app itself, and it's easier interface. Locus seems to also have better route creation than OruxMaps, although not as good as Komoot. OruxMaps' route creation is basic. Both OruxMaps and Locus support Bluetooth and ANT+ sensors, but I have not tested this.

Others
I briefly looked at MapMyRide, Maps.Me, and OSMand, but either their route creation was not as good as Komoot and RWGPS, or their offline map capability was not as good as Locus and OruxMaps.


Conclusion
If I were to only ride with a smartphone, I think Komoot would be the best app to have for quick, on the fly and spontaneous route creation and navigation. However it won't track with sensors. But I could run another app at the same time for tracking, such as RWGPS. If I were going on a long and preplanned trip with only a smartphone, I would probably create the route on the Komoot website (while comparing it with the other map sources of RWGPS) and load it into Locus for offline navigation and tracking.

I would not use only a GPS computer by itself, since they all rely on a phone connection with the companion app at some point in the trip. Although Lezyne has the disadvantage of not being able to save routes in internal memory and always needing to connect with the phone to reload any route after powering off.

If I were to go on a longer trip, I would take both a smartphone and a GPS computer (if I decide to buy one). Because of its long battery life, and being able to operate independently from a phone so that you always have a backup device, I would probably choose the Bryton, even though it lacks third party support. And then occasionally taking a peek at Locus on the phone for points of interests and street names. The Bryton would be fine for pre-planned routes. And for spontaneous changes in the middle of a trip, I'd switch over to navigating with the Komoot app on the smartphone while continuing to track with the Bryton.