Fractal space feudalism?I'm hoping we can make that very difficult actually so that we have strong intergalactic conflicts at space and planetary levels. Perhaps you conquered all of the dry land but the underwater cities still belong to others? Hoping introducing other species can get into surface level conflicts with them on other planets as well.
Obviously, yes, UN as well as conquest is another way to unify into a global governance.

In Crusader Kings you have Empires (galaxies), Kingdoms (galactic alliances), Duchies (solar systems), Counties (planets) and Barons (countries on planets).
First four can wage wars between each other if empire isn't consolidated.
Cultures or religions would be group of galaxies.