Provisioning payment cards on your watch without being able to run the phone app will be quite a challenge, however!
I have never tried this, as I am happier with RFID on my individual credit cards.
However, Google Pay will certainly run on my Lineage OnePlus 5. It will not provision localhost, but I am guessing that it will provision a watch.
I would go buy the parts and try it just to know, but I doubt interest would remain here by the time I assembled everything.
Edit: Graphene has a page on this subject, and Garmin appears to be the best option.
https://discuss.grapheneos.org/d/1040-compatibility-with-sma...
I understand that most U.S. banking apps work on Graphene.
As far as contactless payments, try a Pixel watch. I understand that it is entirely separate from the phone.