My question is, why not use the belt solution more? Evaluate everything that is important/needed/relevant at this point in the games life cycle and delete the rest. And maybe come up with solutions to delete even more. The other solution is they could do what 11 does. Which feels like you vendor the gear to an NPC and then you buy it back from the NPC for what you got for vendoring it.
Or would it be game breaking to have an expansion NPC similar to calamity. You completed X dungeon/raidit unlocks the gear on the npc and for a few gil you can rebuy the piece you want. We can do it with Makai stuff now on Calamity. I do think one expansion behind though. So add the NPC in the X.1 patch. So not exactly convenient or a quality of life thing. You would only have to save Mogstation items and crafted items. And the crafted items just the stuff with obnoxious to get ingredients.
How many months from last real patch to expansion? Roughly 6 months we covered half of your 12 already.