Okay, got a bit further. I'm now in mid-game. Fought three victorious wars, one defensive, two offensive. Gained two vassals, one of whom likes me, the other who doesn't, and cut down a nasty rival to size.
This part is clearly more interesting as I can't just faceroll everybody; digesting my conquests is properly challenging. There's a looot of complexity unfolding now, as I try to keep my unwilling vassals loyal and slice up my too-big-to-swallow rival neighbours into smaller bits. The idea of conquering the entire galaxy seems properly daunting.
I really don't dig the doomstack-vs-doomstack combat though; it's really set up in such a way that fights are always to the death. I thought I was being clever by splitting my fleet into two and doing some fancy manoeuvring to lure the tech-equivalent enemy's doomstack right into mine, but I just ended up losing my diversionary force. So I just ended up beating him into submission by brute force.
I also think the sector implementation is bad. Not in that sectors themselves are bad -- microing planets gets old fast -- but I hate that there's an influence cost for removing planets from sections, and I hate that I can't just make them supply strategic resources to each other. Now I understand how I should've played the sector game, but it's kind of too late, so maybe I'll start another game for that.
But all in all, this picked up a lot from the early game.