Mobile users can be pointed towards alternative clients. Makes the whole thing much more manageable and familiar. I use sync (which no longer seems to be being updated) but there are others. Don't know if there's a desktop equivalent but that might help with UX problems.
Also, re onboarding - many Lemmites might be astonished to realise how many non technical ppl don't know what a server is and are intimidated by the term. Maybe just say something like 'Click on the place nearest to you or that you like the sound of'. I dunno, a metaphor. Like, your computer is a toilet, the fediverse is the ocean. When you take a crap and flush, your turd goes to the local sewage facility where it parties with all the other turds before being dumped into the sea.
There's probably a better metaphor, but, y'know.