Not all banks do SEPA Instant, yeah. I suppose the answer to that is connecting using PSD2 and sending the payment request then checking payment actually went out (using something like https://gocardless.com/). Many countries also have local bank link systems (e.g. in the Baltics it’s common to have dedicated buttons in the payment form for the big 3 or 4 banks everyone uses).

I’m talking about in-person payments though. It would be so easy to implement QR payments backed by the existing SEPA Instant rails. Many bank apps already understand EPC QR codes (usually found on invoices), so shops could just show these to accept payment. In case your bank doesn’t support SEPA Instant, you could show the cashier the receipt in your bank app, which, well, horribly insecure, but probably fine for low-stakes cases like grocery shopping (you don’t want to be banned from your grocery store chain for forging a 35 € payment).