Is there a way to disable the Char planet creation? (And potentially Aiur for Protoss?)
I wanted to try this mod with the Space Age Without Space mod but it fails to load because of missing Vulcanus assets for Char.

are you using that "Enable All Feature Flags" mod?

I was not, I tried it with that "Enable All Feature Flags" mod as well just now and got the same error. Specifically on startup it says in either case:

Error while running setup for space-location prototype "char" (planet): "vulcanus_coal" is not a valid autoplace control name.

Which I imagine since the planet doesn't exist with the "Space Age Without Space" it causes problems when Char is trying to copy world gen settings or what have you from Vulcanus.

The "Enable All Feature Flags" could cause this issue as well.

That "Space Age Without Space" deleted a bunch of space-age entities and ya, that break new planet creation.

Sorry. I won't support mod that breaks/workaround the planet feature. Many new features in my roadmap depend on the new planets (char/aiur).