I'm aware this is not possible for everyone but: if you're in a small enough FC with the right privileges (and the right people I guess), you can make several alts, put them all in the FC, level them high enough to get them retainers, and boom 2 extra retainers (or more depending on your sub) and their inventory slots per alt for you to use, and you can transfer the items between them through the FC chest. I've been using that method when I needed Jet Black Dye on tradeable not-yet-bound gear with Gold Chocobo feathers.
And then you can expand that system even more with alts on other servers, but then you need a friend to trade stuff from one character to another.

... However on my main I've been keeping my completed relics, eureka and bozja gear, and deep dungeon weapons in my retainers and yeah, after 4 expansions and idk how many jobs I'm starting to sweat a little (yes I am aware that you can throw away the relics and buy replicas, but it's still nicer to keep the originals idk)