I found this server called Iperios and it is built around making nations. You can start your own nation with your own government style your own laws and your own ranks. If you do not feel like leading you can just join a nation that already exists.
The server is running the Allthemods 10 modpack on version 5.4 so you have to use that version to join. It is a whitelist server so first you join the discord and ask to get whitelisted. The info for new players is explained there.
When you join the actual server a staff member helps you out. They guide you at the start and give you a beginner kit so you are not lost right away. After that you can go off and play how you want and get into the whole nation thing.
WebsiteDiscord