In Europe? Debit cards. Hardly anyone uses a credit card in a supermarket.

If I'm going out I'll have both a debit card and cash in my wallet. Given that large scale outages of payment networks are no longer theoretical but to be expected, not carrying cash is just silly. Since I have a wallet in any case, I see no point in using contactless payments on a smartphone — I usually don't take it with me when getting groceries.

I have no idea if I can even do contactless phone payments on GrapheneOS, but since everything else works (including my bank's app), I don't care.