the title says it all 😀
Syncthing-fork is a must for battery life. You can get it to sync for only X minutes per hour.
It also helps a lot if you set the IP/domain manually so it doesn’t always have to talk to the relay first.
First, if you can swing it I’d recommend getting a server with a static IP. You can get can get them as cheap as a few bucks a year from https://lowendtalk.com/categories/offers ofc with that they may have more downtime than 5/mo boxes.
Anyway, if you don’t wanna pay you can use something like duckdns for dynamic DNS and get a free domain that automatically points to your new IP whenever it changes. Works well if you’re hosting from home. :)