If you evenly level alt classes, going through each set of type in turn, inventory is never really an issue. Fishing with it's million baits and no bait inventory fix is the most painful, though I find it's a good use of the chocobo saddlebag to store baits, and can be largely avoided by just using leves to level.

Once you max everything at 80, sets are shared between jobs enough that most of my armory is half empty. You can place and retrieve glamour dresser items, storing the "I'm not currently glamouring, but can't stand to part with" items on retainer. If you somehow need more than 2 retainers worth of storage, you could buy additional ones, but outside of hoarding every crafting mat in the game you shouldn't need the space.

For me, retainer 1 is at max capacity, but if pressed I could delete half of it. 2nd retainer holds on to unique items that aren't in glamour dresser or armoie and is half full. I actually have 3 more retainers beyond that, but they don't hold anything at all, and are just money generators with ventures. I do have an FC house, but the 5 Company Chest slots just store furnishings I wouldn't keep at all if I didn't have the house or apartment.