Took me 500 years. Romulans, Cardassians, Tholians, Breen, all UFP members. I had 90 member worlds, which is 2/3 of the way to canon. Yes, my PC had a problem keeping up but I did it. By 2600 everyone was either in the Federation or a protectorate. Only reason not to try it with the full galaxy is that it would probably make my PC explode. Stellaris is impossible to manage with 500 planets anyway. But it can be done!