I have almost no reason to play this game, my only hope now is on these transfers (2.4 is too far away), so I really hope they won't delay it again. I don't give a sh11t about achievements (except my buffs, I want my buffs), I just want to transfer as soon as possible.
I would guess...and mind you this is just a guess...that your buffs would be based on the classes you have on a particular server. So if you had all 4 class types on Harbringer, for example, and transferred a smuggler to Shadowlands, I would think that you would just have your smuggler buff on your new server. That would be my guess, but we'll have to wait and see how they do this.