I can add to this. I've had the same issue on my Skagen Falster 3 (Fossil).
Added my cards using the "Add to GPay" button on the card overview in the bunq app. This works fine on my phone. But when trying to activate the Google pay app on my watch it got stuck saying "something went wrong". Same issue as described here by another Bunq user: https://support.google.com/wearos/thread/14944702/gpay-unable-to-pull-cards-from-google
Removing the cards from Google Pay, adding the cards manually using the card number, exp date and CVC + text authentication token via the Google Pay app directly adds the card in a way that can be transferred to the watch properly. I had to do this both for Maestro and Mastercard cards otherwise they would not even show up as cards to set-up on the watch.
Hope that helps.