My advice is to check the market for all the items youre hoarding up. A lot more of them then you realize are either so common as to be worthless, or are sold on demand by a merchant somewhere. Dont keep either of those types of items, not worth the space. You may think that you want high quality mats, but most of the time you actually dont. The game gives you exp based on how much quality you added to an item during the crafting process, so HQ mats are reducing the exp that you can earn. Use only when absolutely necessary to finish a craft at HQ that youre turning in for a leve or GC provisioning mission. You can either reduce HQ mats to normal quality to halve your number of stacks, or you can sell the HQs on the market to those suckers willing to pay for em.

And maybe im remembering the leveling process differently then how it actually went(i put in the work more then 2 expansions ago), but there werent that many materials required in great number at any given level range. You had a hard metal, a soft metal, a lumber, a cloth, and a leather. Those you needed a lot of. Then you had a variety of oddball mats that you would use a few of here and there. Mass collecting the main ingredients? Fine. Mass collecting the minor ones that only appear in a couple recipes? Waste of bag space. Get them as needed. You dont need mass amounts of mats that you cant work with, and you dont need mass amounts of mats youve outleveled. And im another one who leveled all 8 classes(plus all 3 gathering too) without adding any retainers. I still only have the base 2 infact. I havent really even come close to maxing out my inventory at any given time, so ive never seriously considered adding more.