I'd still be very interested in seeing some support for your assertion that Android NFC only works with Google Pay or the mfr's pay app. I can't find anything. Indeed, Abdroid NFC in settings specifically lets you set a default payment app.
There are two kinds of NFC operations. When you're making a payment with your phone it (which is really emulating a card) is tapped against an active scanner, usually a POS machine. In the other mode your phone is the active scanner and it taps an inactive NFC device (an NFC card or potentially another phone pretending to be a card).
I'm aware that you can change which app launches for the second action, I'm not aware of any way of using third party apps for the first action. What kind of a phone are you using that is allowing you to set the app that provides NFC card emulation?
OK it looks like Android does allow any app to emulate a card and therefore be chosen as the default NFC payment service. I admit I have never tried this but I'll give it a go when I get back to China.