I like Matrix but unfortunately it has major stability issues. The GrapheneOS project moved most of their chat over to Discord after their Matrix community got nuked twice. They still maintain a Matrix community which is bridged to their Discord instance, but most users are on the Discord side of the fence.

IMHO, Revolt is a better FOSS Discord alternative: https://revolt.chat/. Relatively young project, but they are unashamedly cloning the Discord user experience (even with the name). By default it uses infrastructure in Europe run by the project maintainers but can also be self hosted.