So I finished all the available missions, the final one was amazing. You can level past 21, but you can't complete the Subcommander Promotion mission after getting your new ship, that is how they gate you from the other missions.
All the federation system patrol missions are open to Romulans, you have to go to each system individually, you can't get the actual mission wrapper.
All the exploration and diplomacy missions are open for Fed aligned Romulans
I flew past a level 42 Romulan earlier, he must have a lot of time on his hands.