I find the quality very good. And there's no problems with setup a "server" :)
- Go to the preferred Jitsi Meet server.
- Enter the meeting name (as it really called) or just press enter to get a random name.
- Enter your details and options (mainly mic on/off and webcam on/off).
- Enter the meeting. If you're the first one in it, you're the moderator.
Once inside and as a moderator, you can create so called breakout rooms. These works just like subchannels in Discord. If you're many people in 1 call, you can create these to create a more calmer experience. I have personally not tried these out since I have only talked to 1-2 people.
Mumble is a excellent VoIP software, but like you said, screen sharing is not possible since Mumble is just what VoIP means: voice over IP.