I found this modded server called Iperios and it is all about building nations inside Allthemods 10 version 5.4. You can make your own nation and pick your own government style and set laws and ranks or you can just join a nation that already exists.
To get in you have to join their discord first and ask to be whitelisted. After that there is a channel for new players where you can read starter info and get the server ip.
When you finally join the server a staff member helps you out. They guide you a bit at the start and give you a beginner kit so you are not lost and then you can go off and start playing.
You need the ATM10 modpack on version 5.4 installed or it will not work so make sure to set that up before you try to join.
