There has been almost nothing in the game I can't solo. The rare quicksilver mission can be dicy but other than that I've had no problems. And the quicksilver missions are in no way essential to the game plus there's tons of easy ones if you still want to do them.

i wouldn't say "considerable" - there are some higher payout missions aboard the nexus that are hairy/annoying to solo, but there's very little i'd say can't be done by yourself - that said, I've never had fun playing multiplayer content in this game. To say nothing else, my computer shits the bed when more than half a dozen people are in-system

The nexus has a lot of stuff you're gonna want - and that is a multiplayer hub where you'll run into other players, but AFAIK there's nothing meaningful gated behind multiplayer stuff. If there is, i've still got north of 300 hours in the game without needing to touch another human being so vOv