Love my PM9, and haven't tried a P365 yet, though it's likely my next purchase (assuming it arrives in CA sometime now that the CA Roster's requirements have been tamed a bit), though that's assuming the CA required LCI isn't too crazy etc?

One thing that made the PM9 work for me was a grip sleeve, first a generic Pachmayr, and currently a P365's Hogue, which fits the PM9 just about perfectly.

Another thing that helps me is my PM9's LaserMax Centerfire Laser Guard, which whether in use or not it gives me a great place for my support thumb to rest.