tetrahedrizing a mesh

pygalmesh (a project of mine based on CGAL) can do just that.

pygalmesh-volume-from-surface elephant.vtu out.vtk --cell-size 1.0 --odt

https://github.com/nschloe/pygalmesh/#volume-meshes-from-surface-meshes

I found GRUMMP which seems to answer all the needs mentioned in the question, and more...