You should use one with an AMOLED dark mode. Usually the screen uses the most battery and if your phone supports AMOLED it actually turns the pixels off to make blacks, which is more energy efficient. I have Connect, Liftoff, and Jerboa and they all offer it as a theme option although Jerboa calls it System Black (I think that’s the same thing anyway).

I use SyncThing to backup our cell phones to my on-prem server, and then use BackBlaze Personal Backup for a cloud copy.