Battery respecting - private instant messenger

I've always known that SMS are not encrypted and that they are not the safest way of communication. I tried using signal with others all on graphene os, but all phones seem to have their battery drained heavily due to graphene having to run signal in the background for instant notifications.

What do you all use as a safer daily alternative to SMS for mild sensitive info that doesn't drain the battery?

Preferably: trusted, privacy respecting, free, bonus if FOSS