Dune3D uses SolveSpace behind the scene.
I didn't start Solvespace, but Jonathan was apparently in a DIY mode after developing his take on constraint-based sketching. It's also very easy to go from NURBS curves to NURBS surfaces, the challenge begins at boolean operations which continue to be a source of bugs for us. This is really the only option other than OCCT and the code is small and approachable so I try to make it better.
It would, and it has been considered. The sketch elements in solvespace are significantly decoupled from the solid model. That means we could substitute (via wrapper maybe) an OCCT object instead of our SShell class. Then you'd have to change a set of solvespace curves to OCCT curves to make extrusions from them and such. But that would be most of the work.
We do tag all triangles in the mesh with a sketch entity handle for flat surfaces so you can constrain points to a face. I'm not sure how that would be handled. We will also be tagging edges of the solid with sketch entity handles in the future so we can do chamfers and fillets - say by selecting a line entity and applying a modifier to it which gets applied to the NURBS shell. I'm not sure how that would go with OCCT.
But yes I've given a bit of thought to it ;-)
We'd welcome contributions, and it's much easier to contribute to the smaller codebase. I think there's potential for coding agents to accelerate this work since robust point-in-shell and shell-is-watertight tests are mostly sufficient to judge correctness, allowing the agent to iterate; loosely you could define your geometric operation as a function of whether a point should lie within the output region, then ask the agent to convert that to b-rep. I wouldn't currently expect useful progress without deep human effort and understanding though.