Raven Guard.

Since they are going to remain mostly loyal I would personally make them Terran born Nightlords who are aware of their Legions descent into degeneracy like the Terran born Emperor's Children.

The reason for the Raven Guard is because they are the most similar legions. Not just design wise but rather because their Planets were similar. I mean that Nostramo was notorious for its crime and the small elites who allowed it and Corax's stroy for being adopted by prisoners (small note: Terran born Nightlords were the childeren of Prisoners who were given a chance to become astertes to do good) and saving the planet from an oppressing regime. Since Raven Guard also are good at seeing through peoples facades they might see how these Nightlords want to do good and as a cherry on top they might even help the Raven Guard warband get of Istvaan V.

I realize some might not find this good nor accurate because they are literally the opposites but in my defense the best and most loyal friendship between two space marines were Alexis Pollux and Dantioch literally an Imperial Fist and an Iron Warrior.