Nextcloud is amazing. Nextcloud Talk is amazing. Running all that through your basic NextCloud installation is a lot to require. So, you can leverage my Nextcloud Talk High Performance Backend. It includes access to my STUN and TURN server.
This will allow you to scale to “dozens” of users on your audio and video calls. The servers I deploy are in California, so if you live on Mars, latency will be a bit much. If you need the High Perf Backend on Mars, however, let's chat, I know a guy.
The cost is $4/server/month, billed monthly, and subject to my reasonableness.