> Libfive is quite good to check out too

Lots of potential, but IMO not really usable for real world work.

For example, there isn't any (or I haven't found any) simple way to create a clean fillet between two complex surfaces other than blending them and hoping for the best.

Documentation of the API is lacking, except for a bunch of examples in scheme (not exactly the most popular language for CAD folks).

Python API exists, but documentation is ... reading the source code.

Finally, the implicit surface tessellation algos used in libfive are IMO somewhat unpredictable and getting them to produce precise shapes (which is something fairly desirable in mechanical engineering) is hard.