Catholic countries in EU4 follow the inverse ninja law: the fewer Catholic countries there are, the better it is to be one.
The dream scenario for Catholicism is to be a super Spain with Religious ideas that has absorbed all remaining Catholic countries except the Pope (France/Scandinavia/etc can remain independent, as long as they are all Protestant/Reformed) and has somehow won the HRE religious wars for the Catholic side, with several reforms already passed (not Erbkaisertum). That way you are Emperor by default for the rest of the game, with a permanent triggered modifier just for following the 'winning' religion, near-continuous control of the Curia so that most of your abundant papal influence can be spent on boons, and a great CB on everyone.