@several above posts:

This is overall my biggest dislike for both Triple Triad and Blu. I get why they did things the way they did to encourage gameplay back into the main "loops", but I really like when side-content is purely side content.

Like all TT cards being available by simply playing TT. Sure, optional drops, but there should also be actual TT matches to get the cards, even if it's a super low drop.
(On Blu, unrelated, but I really wish everything for it was entirely open world or masked carnival as opposed to dungeon/trials)