??? Blender has scripting but it's for artists... Like, artists make movies with it... Movies that have scenes with dozens of artists working on it...

Yes, but most artists can't script. The scripting is for tech artists or programmers to build tools/pipelines on top of Blender.

The demands of a movie differ from a game which is why I'm a bit doubtful. But nothing is impossible to achieve with peope planning, so I'm not dismissing it.

The people who are used to making models are probably not the same as are building levels.

At least when I last tried Blender the UI was a lot harder to use than something like Unreal or Unity editor.

I'd probably look at the Godot editor first.

But I do like the idea of trying to find an existing editor in the same domain and modifying that rather than building from scratch.