I want to play the cyberspace sections from the remake without replaying the regular sections, possibly make custom sections,
and change that awful final boss fight

Does anyone have some pointers in modding Unreal Engine 5 games, SS1 Remake in particular?

If they (Night Dive) made a game 100% about just the cyberspace, I would play the hell outta that.

Cyberspace as good as it is, is missing quite a bit. Upgrades for player abilities, more open and less linear levels and better enemy variety, would be top of the list.

As for other game alternatives:

* Forsaken
* Descent (several entries to choose from)

I find them pretty average (as someone who likes the remake overall), they are competently done and the shooting mechanics are okay, but they lack depth precisely because they have no upgrades or anything to keep them interesting. Once you have played one cyberspace segment, you've played them all. The difficulty largely remains constant and nothing new is introduced, except right at the end, where it's just awful.

I agree with the previous advice: Play Overload. Everspace and Sublevel Zero are also good recommendations to get your descent fix. Even Starfield has decent space combat if that's what you're looking for.