etrex20 works great. With premium subscription you can write directly into the garmin units from the PC (both my etrex 20 and my older 60cx work this way).
Without direct writing, you download the file and copy it to your unit (as a drive on your computer). Copying the file is easy to do. The direct-write really isn't worth much (don't pay for the premimum subscription for that). I typically also load the courses on my smart-phone, which means I have to download files anyway.
(ridewithgps lets you turn on/off your subscription level.)
That's the Etrex 20.
Just to be clear, you do get prompts with tracks on the Garmin 800/810 (and 705).
RWGPS came to the same conclusion I did where they prefer tracks because they are more predictable.
This is entirely expected.
Because the "route" (the list of major turns) can be so wide-apart, the Garmin has to figure out what to do in between the listed turns/points. What the Garmin decides to do might not match your planned course very well.
Using a "track" (lots of points indicating the detailed path being followed) means the Garmin has to do less (next to none) figuring out what to do.
I have not tried generating "routes" since I switched from garmin maps (city navigator) to RWGPS.
I suppose you mean having the Garmin compute a route. If you are doing a planned ride (let's say an organized century), you wouldn't do that. It is useful if you are trying to get somewhere and don't have a planned route. It looks like the Garmin 800 units can only compute short-ish routes. That is, if you are going a long distance, you might need to do the route in stages (computing the route at the start of each route). A smartphone might be handy here because it can give you a better overview.
Note that you can load custom locations to the Garmins (and have the unit compute a route to a location).
But your comment about having to watch the screen... true but still way easier than staring at a map or queue sheet on your handlebar bag and wondering if that next road is the turn you're looking for and, oops, no street sign, so now what do I do?, etc..
The Garmin 800/810 display the track as a purple line and your travelled path as a light-blue line (other units are probably similar). It's fairly easy to glance at the screen to see if you are basically on-track.
It's even not that hard to keep track of the purple line when you are off course (that does often require some zooming/panning of the device while riding).
And regarding battery power... you might be thinking of smartphones and their terrible problems with gps navigation sucking power. I find the etrex 20 to last a LONG time. Sometimes I forget to turn it off at day's end. The next morning it's still running and has plenty of battery life left. One spare set should last you a week of riding all day long.
The Garmin 800/810 are also very good on battery life (maybe, not quite as good as the Extrex 20). On longer rides, I carry an external battery pack that I can use to recharge the Garmin 800 and my smartphone.
The Garmin continues working while being charged (you do have to be able to put the external battery somewhere and you might not want to do any recharging when it is raining).
Lest I sound like the gps is all "pros", here is one "con"
Sometimes maps are wrong and you need to get a bigger picture to plot your alternatives. These little gps devices are very poor for that. That's when I pull out the smartphone or the backup paper maps (if I'm carrying one).
Any map can be wrong, of course (I had a smartphone mapping program that didn't show a highway that was certainly there).
I, too, use a smartphone as a backup.
The standard mapping/navigation apps (Google and Apple) work pretty-well but they (basically) require access to the cell network.
If you carry a phone, you might consider a mapping program that lets you download maps before-hand. (Note that some of the apps make downloading maps really hard.)
Note that some of the mapping apps also let you display tracks and waypoints/points-of-interest.