This.
Sanctum of Domination is not tuned to require rank 5 or 6 legendaries. Even the folks doing world first don’t have them. The price will drop, right now they’re high because everyone ‘thinks’ they need them, but they really don’t and they’ll become affordable, especially as more of the vessels get added to the market as more players get the items required to make them.
A rank 4 legendary is all you really need right now, so work towards getting that and you’ll be set for a while.