If you want an open-source kernel, you don't have much choice.

Truck and Fornjot are incomplete and not quite ready for prime-time. libfive is FREP, not BREP.

OCCT is the useable of all. But it also has old architecture, all sorts of imperfections (some of them described by the other guy here), and the code quality isn't great, I'm told (by much more experienced people).