I think the best way to make bitcoin a usable currency in everyday spending would be to create a new kind of QR invoice including the (minimum) fee rate and RBF state.

If I were providing a service, I would definitely accept 0 conf transaction on the fly providing they are without RBF and within the first 1MB limit in the mempool up to at least $1000.

Ideally the customer would be left with 2 choices:

Pay without RBF the recommended satochi/bytes and you can leave with your coffee to go.

Choose a smaller fee am/or RBF and we wait together for a confirmation.

But I only see it working if wallets start accepting a new kind of QR code enclosing the fee rate.