You don't have to sideload lol just use obtainium or f-droid. The full FOSS version is available.

iOS and windows... I mean, if you're still joyfully locking yourself into a proprietary walled garden, that's on you. Out here in freedomland its not an issue.

XMPP the protocol is somewhat dated and has some issues. But to this day it's the best messaging protocol available, ive tried them all.