

Last time I ordered a phone battery from AliExpress I got it in two weeks flat. If you notice that your battery doesn’t hold as long, just order a new one, it will arrive long before the device dies. There are also local battery shops, but they will charge a premium for quicker delivery.
Doing this once every few years is nothing compared to the hassle of taking out the batteries every time you want to charge them.



I’m sure you are already aware, but just in case, there’s a lot of prior work in getting a truly Linux mobile phone.
There are ready-made devices like PinePhone (the PinePhone Pro looks the most promising one of the bunch), Librem 5, and Liberux Nexx. I think at least some of those companies publish schematics for their boards, you should probably check those out if you want to design your own.
There is also another direction, taken by postmarketOS and the like, to install Linux on a phone that shipped with Android out of the box.
It should be easy enough to install postmarketOS on your device, since it seems to have support for raspberry pi. The benefit of postmarketOS here is that it makes it really easy to install mobile Linux UI shells, like phosh, gnome-mobile, plasma-mobile, or sxmo. This will let you try all of them out and maybe pick one as a starting point for your software stack.