Oh well, no luck with Gmsh either! After having successfully generated a file.msh with the 3D geometry I wanted (same as above with mshr), I cannot work with meshio because of an already reported h5py error message about a version error. The solution proposed at Gmsh 4.4.1 in FEniCS? Meshio to uninstall meshio and h5py and then to reinstall them but h5py with a special command, does not work. It produces a gigantic error message that I reported in that link above.
Thus, I am left unable to work with any mesh generated by Gmsh.
So, I am completely lost after all these efforts. I cannot seem to work with neither mshr (not fixable because it’s too limited) nor Gmsh (due to the h5py problem) at the moment. What are my options left?