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IMO, extra retainers are not really worth it.
I'm crafter too, and I still manage quite well with 2 retainers only. And since all my gathering classes also capped and quite well geared, I consider every gather node as my endless supply of mats, unless I'm really lazy to travel and gather. lol
Just hint from mine though: Don't stock up those culinary/alchemist mats because they are one of big reason why you never have enough space. Just store what you really need the most, like for end-game raid needs or doing high level crafting/gathering.
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I keep an entry level account (8 characters total, 1 per server) and use the two additional retainers. I use 1 for gear, second hold/sells the things that go towards my FC's funds, third is materia and fourth is crafting