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.

https://www.youtube.com/watch?v=-rUminiQjtM