Is there any way to put the aux features into the vanilla auction interface like auctionator on retail does it? I'm really not a fan of the flat grey boxes.

no.

sorry, I'm finally happy with the current one after having gone through like 3 complete rehauls and it's a lot of frustrating, boring work to create GUIs

Could a new GUI be created then? It really is an eyesore with the rest of the vanilla interface haha