AddInExpress, instead, claims(didn't try it for myself) such process to
become much easier. I haven't tried them, but if they only ease the
process by - say - 30% that's surely worth the value.

The most complex part of creating such add-ins for Office is the fact
that there're *awful* COM complexities to deal with. Just adding a
toolbar can be a real PITA, because you have to take care of so many
things, avoiding to mess with the "Normal.dot" template and such beasts.
Plus, while the previous run it seemed to work, the next for some reason
might not work the same way :-( .
It's one of the worst programming tasks possible.