In terms of the “default instance” suggestion, I have an interesting hybrid suggestion. What about having an “easy on-ramp” instance where you get registered for one month with a hard-exit (auto-migrate to other instance, perhaps using some kind of federated-auth/token system for the migration, and forced password-setup on first use of the new instance). At any point during on-ramp the user could configure destination-instance from a list in the settings (or configure auto-export for manual import to any other “auto-migrate-unsupported” instance), with optional early-migration if the user has decided before the end of the month. Optionally a recommendation engine could iteratively curate a list of suggested instances based on usage during on-ramp (admins of those instances could provide - limited number of - tags of their choosing for the engine to use for matching). That part could be opt-in because probably a lot of users would find it creepy. The UX would need to be very user-friendly “pointy clicky” because that would be the overwhelming target demographic of such an instance.