While I get where you're coming from with the possible cooldown, lore-wise Aegon conquered the seven kingdoms (six, I guess, not Dorn) in a remarkably short period of time. Adding a 5yr cooldown would make that conquest take at least 20 years, so it simply wouldn't work in key scenarios like that. If there were some way to limit it to continents, that might help.
And while dragon conquest is over powered, you have to remember that unlike an invasion CB everyone you're fighting retains their titles. So succession will be a nightmare, as a whole bunch of kingdoms are going to rebel.
And while dragon conquest is over powered, you have to remember that unlike an invasion CB everyone you're fighting retains their titles. So succession will be a nightmare, as a whole bunch of kingdoms are going to rebel.