Here is what I've done...
1. I will use an outside app, RideWithGPS or Komoot to create my route.
2. I then do some form of "send to garmin" type command.
3. I use Garmin Connect to synchronize the GPS and the device (and also retrieve the resulting activity)
That works well enough for me.

I think people can have different preferences, but where I've found my preferences...
1. On the PC, I've mostly done RideWithGPS including the heat maps to draw routes, that works as well as anything.
2. On my phone, I prefer Komoot if I need to create/re-create a trip on the fly. It has those different modes (e.g. road cycling, general cycling, mountain biking) that seem to pick default routes that work better for me than RWGPS or otherwise
3. Garmin Connect seems to be a good overall connecting point - and also synchronizes with Strava, which I've let results get posted
4. I *avoid* using the on-the-fly route finding my Garmin has built in. Sometimes I'll get there in a short distance if I've missed a turn but otherwise it seems to be worse than other choices.
5. I keep an offline map (e.g. maps.me, osmond or in my case organic maps) so I can make some quick map checks when I am outside cell service or sometimes just as convenient
6. I will also occasionally check Google Maps when I have service - generally better than my offline maps but I prefer Komoot if I have to do any routing on the phone and RWGPS if I do routing on the PC
7. I like paper maps to spread out on a picnic table and get an overview.

In any case, I find I use a mix of tools for both online/offline, PC/phone, device/non-device navigation - this also lets me try and change different methods as I go along. For example, I found some seem to work better in populated areas and others in more spread out places so I'll try and mix/match them. But in any case, Garmin Connect works well enough to synchronize my Garmin device and my phone or apps.