My family has been using Synapse since before Covid hit, including TURN server (coturn) for audio and video calls. No complaints about the UI so far, except for Element on iPhone reportedly “freezing” for a few moments when sending messages - and that was over a year ago.
What problems do you have getting TURN to work?
Mastodon does not need to scale well. Even if the userbase shrinks to what it was before the Big Twitter Exodus (or whatever you want to call it) of last year, it will be perfectly alright.
Mastodon was fun and enjoyable before. It does not need more users to remain so.