I have a lot of experience with these things from my back of nowhere off road motorcycling. It's really hard to beat the Garmin eTrex series which GPS City have on refurb right now -
Advantages are running on AA batteries which, if you don't have a charging source, can be bought anywhere, the bike mount seems to be very sturdy as is the unit overall. You can find deals at GPS City on the older models too, I think all you'd be giving up there is the 3-axis true compass (works while you're standing still).
Garmin are almost the only choice out there now, but you don't have to use their (expensive) maps as quality 3rd party free & paid products that are compatible are available. Warning, the Garmin Basecamp software has a steep learning curve & some idiosyncrasies. I design my routes in Google Maps then export to GPX and load from that into the Garmin, also a lot of the latest route planning apps will upload directly to the Garmin units removing Basecamp altogether.
Another option that you probably already have in your hand is your phone & offline maps. There's a lot of confusion around this but the bottom line is, you do not need to have cell service to use these. You can again route plan in Google Maps (or any number of other apps) and download the map area you will be in for offline use. The phone GPS sensors work irrespective of a connection or not.
Disadvantages to the phone are battery life (easily solved with a portable charge pack), difficulty to see in bright light and in hot climates, over heating in full sun (happens in Texas). There's a ton of bike mounts from Amazon no-names to the great products from people like RAM Mounts.
FYI; I have no affiliation to GPS City, have just bought a lot of refurb units from them over the years & been super happy with them & their service.