Since you mentioned ssh and vi(m), I’ll throw out vimwiki. No Android app, unless you count termux, and there’s a bit of a learning curve, but idk, it really clicks with me.

As an old man who has clocked 150 hours, yes, it will. :)

My 3rd gen Moto G had a removable back and was also water resistant. Though I don’t think it had a removable battery.