There is already the EPC QR code, which contains all the data required to initiate a SEPA credit transfer. This code is supported by practically all banking apps (at least in Germany). The standard is public and free (see
https://en.wikipedia.org/wiki/EPC_QR_code)
The merchant's system displays this code, you open your online banking app, scan the code, select "SEPA INST" (here's the usability catch!) to make the payment instantaneous, and confirm. Within 10 seconds, the money is transferred to the merchant's account.
Either the merchant's bank or a third-party Open Banking API immediately informs the merchant's system (e.g. by push notification or webhook), and a receipt is issued.
Everything is already here, but since this system would be virtually free to use, nobody really has an incentive to push it. It costs money to educate the public, and there is no money to be made. Instead, everyone gets paid handsomely by the card mafia.