The larger aspect is effectively restarting repeatedly on new worlds which have unique new resources until they too are shipping rockets of stuff into space. Each new world has different properties. Some have almost no solar, others no water, some have no bugs, but others have bugs dropping in from space anywhere on your base so it's not the same thing each time. I don't mind this aspect. It is a bit like playing Factorio on repeat but it has reasoning behind it. And each restart you get to bring a cargo rocket of stuff with you and the ability to build anything you've already researched.
My outpost worlds are often messy and do the bare minimum to send up rockets of stuff to space. But it works. I'm finding it fun but I'm only a couple of hundred hours in.
Completely different. The SpaceX mod used as an endgame in Seablock has no relation to Space Exploration.
Space Exploration makes pervasive changes throughout the game, and adds the ability to launch yourself into space, build a space station, build colonies on other planets, and mine resources on other planets (and elsewhere) which aren't available on the starting planet. The research required for the first rocket launch is reduced, and there are multiple tracks of research unlocked once you get into space.
I'm willing to be told I'm just doing it wrong, but does vanilla + Space Exploration have a different sort of challenge curve, or would it end up being pretty similar to Seablock (which includes SpaceX, dunno if that's the same or different).