Collecting the cards feels bland, you just click "buy 100" until you have the card. If you run out of money, you wait until enough money has accumulated. Having goals like "have all cards with a rating of 10" does not make it more interesting, it is just more waiting.
Arena: I played 100 levels (using an exploit, but that's not the point). From a design perspective, the arena needs fundamental change. Any good game - except reaction games - is about interesting decisions. Or in thee words: Incentivizing interesting behavior (https://www.boardgameauthority.com/gil-hova-game-design-theory-self-publishing-s...)
In the arena, there is only one interesting decision: Buy an item now or wait until something better comes along.
But this decision usually doesn't exist, because most of the time the items offered are useless (e.g. extremely weak items, like stones when I have lasers,, 2x heal 100%, item with +1% Crit).
So make the items offered more useful, then the player will always face the decision to buy now or save up to buy something even better.
Healing option should be available all the time, as it's a no brainer to heal. Just an expensive full heal option is enough, as the interesting behavior would be to risk death, to save on healing cost.
The initiative and the crit chance increase should be incorporated into other items, then you will have more interesting decisions which items to use (e.g. heavy armor gives good protection, but decreases initiative; small dagger does little base damage but increases crit & initiative).
If all your characters would attack (and get hit), raising initiative and managing health would be more interesting.
In general, I strongly recommend to change the design perspective from incremental (repetitive, no-brainer) to decision-based (choice, tradeoffs, incentives, impact -> player agency). Look at all your systems with that "lens", and think about how you could introduce interesting decisions.