*my suggestions are usually based around a low development time with high impact because i'm very aware of how both of these things work, this suggestion won't be low development time but it will remove quite alot of frustration due to how the brain works, knowing there's an alternative to just pure luck is an insane thing and gives people the will to actually grind.
As many players know, the dreaded RNG is both very needed and frustrating at the same time in a game of this genre.
My suggestion is: Add different currencies when you deconstruct certain items from different places, i will explain:
Something similar to the trials "Tower of the mad mage" and "Zariel's challenge (Master)" currencies, they have done this in a very elegant way, i've heard the "oof i ran 40 runs and didn't get a single weapon, i just bought them with the currency" quite a bit of times.
When you discard undermountain companion gear you get "comp gear currency", which can be used in a store to buy the companion gears you need.
When you discard watcher weapons you gain "watcher currency" which can be used to buy different watcher weapons
When you discard a forsaken piece from avernus treasure map you gain "treasure map currency" used to buy different pieces from there.
and so on
When you discard an underdark ring you get "underdark ring currency" which you can use to buy those cursed +5 rings, after all these years i'm still missing quite a bunch and the RNG gods will just not have it with me.
A similar thing could also be done for TONG (get x currency in tong to spend on the now retired primal gear), for CR (with the barovian lord armour AND sun weapons), for Svardborg (for the Jarl's gaze) etc
I am very sure you can make the case for alot of old items that may be good for some whacky build or has a very pretty transmute.
I'm not saying we should have items on a silver platter, no no no, i'm the one that advocates for the grind usually. The price of store items should be very high, but should give you the option to eventually get it if you pissed off the RNG gods. Say balance it at around the equivalent of 10-20 hours of farming for said currency
Potential downsides i see to this would be:
-the development time since there's quite a few new currencies involved and there would be some research needed in order to calculate the prices (unless you want to use an old currency and a random price, which would circumvent some of the grind and be detrimental to the game overall)
-the amount of stores and currencies would be quite overwhelming but due to the way campaign stores work now (quite well i'd say) a fix to the "overwhelming" would be to introduce those things in the old campaign stores, just in a separate tab