Two things that would definitely massively improve first-time-user-experience are
Better community discoverability / joinability.
Maybe having the Lemmy instances advertise the communities they know about? Allow communities to opt-out of this discovery process? It's could be kind of like /channels list on IRC.
Maybe add a "subscribe" quick-button next to links that lead to known (by the instance) communities? That way the friction-to-subscribe is way lower
A way for an instance to "pre-subscribe" users to certain communities by default - maybe even as part of a "user setup wizard" wherein the instance owner can curate a list of communities, and the user that's signing up can one-click-subscribe to all, or choose which ones to subscribe to, as part of the post-registration journey.
Totally food for thought there, and possibly low-hanging fruit to improve UX massively. The initial experience is painful on a small instance that doesn't have many known communities yet.