in all seriousness, using a phone full time on a handlebar aint a good idea, for all the reasons you mentioned.
I do get wanting to use gps technology for some directional aiding, but honestly (and its not just cuz Im an old fart in my 50s) the vast majority of the time we dont need turn by turn directions. Sure, in urban settings it is a help, but the vast vast majority of the time bicycle touring, its "ride along Rt 10 thirty kilometres until you get to Maryville, then turn right onto Rt 22 which is the main drag in Marysville, and then stay 25kms until you get to Jonesville, and then stay on Rt 22.........this sort of thing.
so having a phone, which is an incredible useful tool, phone, webbrowser, camera maybe, gps unit, blah blah blah, sitting out in the sun on a mount that may one day loosen or something, just isnt a great idea.
One option is a waterproof handlebar bag, and using a genuinely waterproof phone holder (I know Ortlieb makes one) so all the time you dont need to look at your phone, it can be in the easily accessible handlebar bag, completely protected from any rain, and if you need to look at your phone, bring it out and attach the waterproof phone holder to the top of the hbag.
To me its a no brainer. You have this goal of doing a gazillion kilometer bike trip, so having a 100% waterproof handlebar bag is completely worth spending your money on. Same with a properly waterproof phone case, one that can be securely on your hbag also.
On a long trip, a top quality waterproof item like an Ortlieb hbag is totally worth it, from the waterproof aspect, to the simple fact that you need a hbag that is easy to remove, to take into stores with you , with all of your valuables inside. You do this EVERY time you leave the bike, no exceptions.
Look into their products, some stuff is worth spending the money on, and in my opinion, a waterproof handlebar bag is one of them.
You may not think so, but you're going to be riding in rain......
also look at it this way, having your phone off or not working its little battery away all the time is a huge advantage, both to general phone life, but as you say, not being cooked in the sun constantly just makes sense.
the nice thing with a well designed hbag is that it takes 2 seconds to pull out a camera or phone or whatever. Add in being able to mount the phone in view when you want to, knowing that it is totally waterproof in a case, is a total bonus.
have you ever toured before?
good luck with planning , and also, good luck with sorting out your health coverage details.