I found a modded server called Iperios that runs on Allthemods10 ATM10 version 6.1.

The main idea is nations. You can make your own nation and pick your government style. You can also set laws and ranks for your people. If you do not want to lead one you can join a nation that already exists.

To get in you join the Discord and ask for whitelist. After you get accepted a staff member helps you start and gives you a beginner kit so you can jump in. You need the ATM10 modpack installed on version 6.1 to play.