So in Dynamis, you really need hundred piece currencies -- 100 Byne Bills, Montiont Silverpieces, and Lungo-Nungo Jadeshells, then a single 10,000 Piece currency for the final stage.
You can farm the hundred pieces directly, though it's pretty rare. And of course there is nothing that drops the 10,000 piece. Instead, people mostly farm and buy the single piece currencies, then trade those 100 at a time to one of the three goblin NPCs to turn into a 100 piece currency. Eventually you trade 100 of the hundred piece currencies in to get the 10,000 piece. In the end, you have something like 170 stacks of single piece currency compressed into 4 item slots. You could gather all your currency before even starting on a relic, and it would only take up 3-4 inventory slots, which is hardly a strain at all (this is in fact what I did when I built my Spharai).
Mythic builders don't seem to have that option. Now I could be wrong, but it's my understanding that you have to gather the Alexandrite for a mythic and store it with Paparoon until you get 30,000. It's impossible to get any of this back to resell (in case you eventually change your mind), and because 30,000 alexandrite would take up 304 inventory slots, you pretty much have to store it. No tap backs. No cold feet. When you're in, you're in.
So how about giving the option to hang on to the alexandrite until you're ready to commit? Add 2 items: something like "green alexandrite" which is worth 100 alexandrite and "red alexandrite" which is worth 10,000 alexandrite. Players could trade up just like they do with Dynamis currency. Or you could even let these things drop (uncommonly) from certain things.