In my country, New Zealand, Garmin pay only supports cards from one bank and about five random credit cards issued by stores and consumer credit firms. It is exceptionally disappointing.
In the UK the situation is similar, but that didn’t seem like a major blocker to me. I just opened an account with the supported bank.
On the plus side, it does seem to be widely supported at payment terminals (I’ve yet to find anywhere that accepts card payment that doesn’t accept garmin pay)