Hello, O'Shea, and welcome to incredibly complex world of imaginary governments!
At this point in development, the question of 'which kingdom is right for me' is an incredibly difficult one to answer. I'd recommend checking out the Kingdom and Duchy Master List and using the information to look into various Discords. Think a little bit about what's important to you and see if you can't question Dukes and Kings on just that!
Personally, I'm a a proud countess of the double-duchy, Zylphania, who in turn is a part of the double-kingdom of Vornair. I made the choice a year ago and my only regret is not choosing to pledge a higher package.