just enough to visually distinguish two otherwise visually similar cards.
How about a simple, old fashioned text label for each card?
For all I know there is something like that, but it'll be buried in settings, probably in accessibility, where nobody ever goes.
Discoverability of options on the iPhone is fraught with danger and distrust, I learned about CarPlay widgets a few days ago and I've used it for years.
I've tried to come up with the reason they don't do it, and thought MAYBE they were afraid people would put PINs for the cards there and they thought it's a bad idea. I wonder why I even tried to invent a good reason for them, designers could just lack perspective and do not care at all, even in apple.