I’ve heard people say you should start a new game. Which I did. But I think you’d be ok to jump in if you wanted. There’s a new neighborhood that it mostly takes place in. To start it, you have to complete some main game story mission about 3/4 of the way in, if I remember correctly.
I’m torn. More For All Mankind is good, but the Soviet stuff we’ve seen already is a big bummer for me. (not to get into spoilers) Generally I like a paranoid thriller, but not if my faves are tied up in it!