alt Hacker NewsMakePass is an app that creates independent/proper Wallet *.pkpass files. You can completely customise the pass appearance according to Apple’s specs.
(Not sure whether the current licensing offerings are worth it, though. I’ve got grandfathered in from before it had AI support.)
This looks like it would work well, but it’s a subscription. It makes no sense to charge me a yearly fee to make one pass file for my library card.