You know the UFP is based on Idealistic principles, if it isn't perfect, it won't be acceptable.
That's probably why the UFP only had 350 members at it's peak.
It's also because of its principles and ideals that it endured for so long and inspired so much hope and people who still want to be part of the Federation (despite the Burn).
Those principles and ideals also helped Voyager crew cope with their isolation in the Delta Quadrant and apply them in practice on some occasions (when situations allowed) with positive results.
Also 350 planets could easily encompass majority of the Milky Way galaxy (with 'pockets of space belonging to some indigenous 'minor' species that were once part of larger organizations. For example, if Quonos joined the Federation, I think most of Klingon space would automatically become Federation space.
If the Founders also joined the Federation at some point, the majority of previously known Dominion space would become Federation space automatically.
I imagine that 'minor species' which were part of larger organizations like the Klingon Empire and Dominion would be considered as 'previously occupied' (same as Bajor) and be provided Federation aid to restore themselves and then offered to join the Federation if they wish later on (and much like Bajor, I imagine that most would).
Would be interesting to see how much of the Galaxy is considered 'Federation' with all 350 planets though.
Now with the spore drive, it would become a lot easier to maintain contact, etc.
Some things from this episode:
Starfleet didn't manage to replicate the Spore Drive (which seems a massive stretch if you ask me, but fine) and Osyraa knew this... which only further supports my supposition that Osyraa could have easily taken Discovery to Emerald Chain HQ, and study its spore drive to give herself a massive advantage over the Federation.
The notion that replicators convert energy into matter was retconned to now say they simply break matter down into base elements and reconstitute them into different forms of matter (or the replicators of the 24th century really DID convert energy into matter and because of the Burn and relative scarcity that ensued, the Federation had to resort to matter to matter conversion technology instead - also, I don't think they need fresh form of matter because the technology in the 32nd century would have to be massively more efficient where losses don't really occur - heck, 0 waste is essentially a policy for the 24th century... so 32nd should have the same one - a fresh supply of matter isn't necessary when the crew's excrements [in whatever shape or form] are probably harvested [which probably includes dust, etc.]).
Zora is finally a prominent character - yay.
I think she will be instrumental in helping the Federation replicate the Spore Drive and using non-human interfaces in Season 4 and onwards (which will probably deal with previous and new Federation worlds joining back together).