You don't need a legacy buff; I don't understand why a common sentiment in this thread is that transmog needs to be gifted to you by a 500% damage multiplier.
Opulence was soloed on mythic last expansion, and it's only getting easier with more gear. You can kill one golem solo and do the boss with a +100% buff - given that this was doable on normal / heroic last expansion, I wouldn't be surprised if you can zerg the boss ignoring both golems with some vault gear.
As for Rastakhan, you will need to hold everything for the burst window after the transition but before bwonsamdi kills you. I think you can use an afk alt in some room to do it normally, too, but not so sure how this works. (I'll probably look into that soon)