Okay this might not help you much, but here's how I think about it after a few years as someone who really was super into the 'what if I need it later' mentality.

Price check it. For me any item that's less than 200gil is not worth keeping, chances are these items are readily available and won't 'run out', the same could be said about every single regular crafting material (that's not timed). Fishing can be abit tricky as well.

Low level materia are worth keeping if you plan to do ARR relics, it requires you to meld alot of low level combat materia and some of them can be like 2k+ ea which is alot, they're not readily available since they are a result of extraction on low level gear.

Dyes, are all worthless with the exception of jet black and pure white which go for a high price.

Relic items, worth keeping but remember if you're going to do all the weapons, expect running into inventory/storage issues...but since you're limited by poetics, I'd say the storage issue is maybe worth it (for me)

Faded Orchestrion, it depends really, chances are, come next expansion, people will be running these trials unsync and they'll become worthless for the most part. Do you want to sell orchestrions? If so, keep them and use specialist on Alchemist.

Souls for crafters, I think you can only get specialist for two crafters at once, it's really niche imo? I only have specialist on CUL/ALC, the soul is fairly easy to get as well, I think a couple hundred scrips? 300? Or so.

I think at the end of the day, it's up to you to decide what's worth it or not. I personally only keep mats for stuff that I know am going to craft and sell the rest. I keep materia around because I like sorting them and they look pretty. I personally hired 2 extra retainers because I didn't want to pick and choose, the storage from 4 of them is plenty and I hardly even use the storage of the last two.

I don't think we'll get more space/storage/material/crafting bags so long as retainers are being sold as a service tho