I suggest adding a UI item, either above Gold, next to Gems, or next to the player name. This would track a single inventory item chosen. If cleared / nothing chosen, the element can be hidden, or a button like “track inventory” can be displayed. Any single inventory item can be chosen, and the item Icon + current quantity should be displayed. If a button is not shown, there can be a “Track” button instead added in the Player inventory next to all items not currently shown.
Alternately, all or some of the current 5 items (Gold, Glory, Gems, Souls, Medals) could be selectable to change to an alternate inventory item.
During grinds for traitstones, I find myself going back to the troop or class repeatedly to check my progress. It would save time and annoyance to be able to see at any time what my status is. It would be much more complex to add the tracking to troop traits, and I can write down or remember how many I need, so just a quantity would be a good start.