As much as I hate PayPal, this is probably your only option for NFC payments on GrapheneOS then

Gaël Duval, the lead developer behind /e/OS, has been recommending an app called Zettle on his blog. It at least work on degoogled /e/OS, probably GrapheneOS as well. I have no personal experience as I prefer sticking to cards.

The most famous one is probably PayPal. But I assume for people who are privacy focused, that would be not the first choice.

There is Curve Pay, but I know nothing about it besides the fact that it exists.

Samsung Pay exists, but it only works on Samsung devices and therefore is not really an alternative in this case.

And then there are some regional options. Here in Germany are same banks that offer their own payment apps, most famously the Sparkassen. I heard there are also some Indian and east Asian payment apps, but I don't know much about them.

In a lot of European countries there are local payment apps that work as alternatives. Some banks even have payment support in their own apps.

Check your local market for alternatives, though if your market is the US then there probably isn't any.

Curve pay allows the creation of a card linked to a bank card