[Sapient]

There is no reason for there to definitely be that ONE 'colonize everything' type civilization.

I would assume that it would be incredibly common for a species capable to interstellar travel to have 'cured' aging before starting to colonize the universe. There is no way we can know how a being with eternity before them thinks. Perhaps they will just do it very very slowly? Perhaps immortal civilizations have very slow population growth and decreased pressures to colonize? Perhaps there is a lot of red tape in the Interstellar Colonization Bureau to go through before you can colonize planets? Perhaps the ONE civilization decides it can only colonize barren planets, in which case they have to terraform before they can settle, slowing the process of colonization down exponentially?

I really love this topic, there is so much room for imagination.