Can't say it any clearer.
Raise the stack limit on materials (Spirit Bloom, Relic Iron, etc.)
If you say you can't increase the vault size, then at least make it so we don't have to waste space on 200 limit stacks of mats... Weapon Parts, for example, don't have a limit, why mats???
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I've been hoarding because I'm hoping at some point there's an exchange setup where all materials of all kinds are available for purchase using other materials as currency. But that wouldn't make us grind more, which is what the desired outcome is. So, likely, this new exchange would use faction marks as currency.