I have been looking into self hosting a Matrix server. Call and stream quality seemed pretty good, however there can only be one person screen sharing at a time. So yeah, unfortunately Matrix won't work as a discord alternative unless they get an update to allow for multiple people to screen share.
I remember there was a Discord competitor called Revolt that recently changed their name to Stoat. Haven't heard much in particular about it, but does any one have any thoughts on Stoat?
Edit: you can view multiple screen shares at once if you use Commet