Not sure that finding an alien biome that could support a species is likely. Unless all life in the cosmos comes from a common origin or there are only one atomic and stereoisomeric pathway to life it is highly unlikely that any alien biome is compatible with another. As for terraforming PBS Spacetime has a interesting video on terraforming MarsI wonder what takes longer: finding an alien biome that could support your species, or altering a "close" one to fit.
This, in my opinion may well be the biggest obstacle.Convincing civilizations that have spent a hundred thousand years aboard a generation ship to leave it and settle on a semi-hostile planet.
If you have the tech to get there you may have no need of planets.