That UI flow doesn't exist for SEPA transactions, which is exactly why these systems came to be. Some of these systems already existed before SEPA came to be in 2014, which necessarily made them national systems (before 2014, cross-border transactions used to be complicated and cost money, even if both countries used the euro).
IIRC, SOFORT used to just ask you for your banking credentials, and then it would just log in to your online banking and put a regular transfer through.
Amazon.de does a nice job by offering direct debit: you put in your IBAN and they automatically debit your account, which works across SEPA. But this seems very fraud-sensitive (you could put in anyone's IBAN), which is probably why it's not more common.