it's possible to get a closed-form solution for the pull-in voltage of a
cantilever beam for every geometry (or however for a wide range of
geometries) (that is, beam length, width, thickness, and gap) and
material properties? or it's necessary to resort to some simplifications
(for example, only small angle bending), that are acceptable only for
some geometries?

I've read the references advised in a previously posted mail about the
pull-in voltage of a cantilever beam, but they don't solve my problem...
Can someone give me an advice and some other references?
Thanks for your kindness,
Riccardo Montagnini