That interface looks boss, really good for a first release, looks like its going to be a breeze to maintain this app. Something interesting to keep you busy would be maybe looking at using Siri to change the card.
You could have a in app toggle for the complication to either “Open Curve App” or “Cycle Cards”
so users can choose a different payment card by just a quick tap!
However if you cant change the card via the complication by tapping it then you could add Siri Shortcuts so for example “Hey Siri, Change my curve card to Natwest”.
It might be more complicated and probably wont add any benefit but i just get these ideas in my brain . Ill leave my thoughts there now