Wouldn't another potential solution be that they can remove the 'Unique' modifier on sets of gear? This would allow someone to have an entire other set optimized for another materia set, granted it might suck to have to buy the same pieces, but I think these players might appreciate the ability. I personally got every piece ahead of a few expansions for BiS and that's where I encountered this issue most often, do I'd like the ability to say least.

This would have the con of players that might need on gear they already have, but this could be limited by still locking e.g. alliance raid gear, where the actual raids have swapped to a coffer system so it's entirely within the hands of the players at this point where it really counts anyways.