An actual spiritual successor to KSP is KSP2 Redux, a community project to resurrect KSP2. They made tons of progress already: https://ksp2redux.org/

Kitten Space Agency is being made by one of the studios that attempted to land the contract to make KSP2. Basically, KSP1 had fundamental engine limitations that were blocking adding stuff that the developers really wanted (interstellar travel, better colonies, etc). So instead of just developing more DLC, they decided to make KSP2. Since the company that made KSP1 wasn't a game developer (they were a marketing company that gave their devs some free time), they shopped around for a studio to take it over. Of the studios that offered to take on the project, they ended up choosing Take2, because they had a flashy, art-focused pitch. Take 2 muddled around in development hell, then decided to abandon the engine rewrite, and focus almost exclusively on releasing a graphics overhaul of KSP1, all while still promising the features that required the engine rewrite in the first place. Queue absolutely disastrous early access launch. Rocketwerks, the KSA studio, were another one of the studios that pitched for KSP2, on the technical basis of their proposed replacement engine which would actually solve the problems that was KSP2 raison d'être. After Take2's KSP2 failed miserably, fired all it's staff, and then pretended that KSP2 wasn't dead for months, Rocketwerks announced that they were going to build their own game, KSA, on the engine that they had developed for KSP2.

In short, KSA is more of a KSP2 than what Take2 gave us.

KSP2 modding is a dead end. Also, it forces people to buy an abandonware KSP2 to play the mod. I spent myself those 50Euro on KSP2 when it was still active on development, but now I hope no one else makes the same mistake.