> that every middle school student can pick it up and design things.

The Solvespace UI is a long, long way from being the sort of UI a contemporary kid has any kind of comfort with, I'm afraid, and will be obtuse even to teachers (many of whom, with subjects that concern technology, do not have time to develop expertise in an obtuse UI and may indeed only be confident they understand the meaning of all the lessons they are teaching and not much more).

I don't think bugs in your booleans are your biggest problem at all.

I think Tinkercad has weaknesses as a classic CAD package, and there are things I would like to see done better, but as a package to teach younger people how core concepts in 3D modelling (rather than the ontologies of bRep) actually work, it is the standard you are working against.