Comment by jakecopp

For cross platform chat based on Matrix also see https://www.beeper.com/, built by the pebble founder. (paid service, works with iMessage even on Android).

If you want to self host a Matrix server complete with all the bridges see here:

https://github.com/spantaleev/matrix-docker-ansible-deploy

Or - you could just ask your friends to download Element! https://element.io/