how much more steps does a final user need to make in order to complete a one shot payment (vs CCard) ?

With instant (bank) payment coming in september, SEPA might be the option of choice in EU (also Paris here ️)

At least here in Finland SEPA direct debit is usually disabled by default for extra security* as domestic shops do not use it (we have redirect-based bank buttons instead) and with foreign ones one usually uses card instead.

So not really an option of choice in entire EU unless something changes - you don't want to tell your customers to enable something on their bank account first before purchase.

But VISA/MC debit cards are extremely common here, so I guess card+SEPADD might cover most people in EU. Though I think many people here are vary of entering their card details online as they are not used to that (then again, I guess the advent of Netflix and others must've made more people familiar with that).

* IIRC you just need the user's IBAN and consent, and IBANs are not really private as people use them to make payments to each other.