I only tried to explain, why bunq.me's card number field doesn't support letters, and if it's an issue with Dutch cards, that there is iDEAL as an alternative.
I still don't really get, why you expected support for letters though, I think there is probably no debit card number with letters anywhere and as far as I know most Dutch debit cards don't even have a card number printed on the card.
If you mean the IBAN (NL..), it's not actually the card number. Maestro cards actually have card numbers (but also not with letters), but sadly not all banks are printing them on the card or support online payments with it.
Berend-Purple-Wolf With true SCA, the merchant or PSP would support EMV 3DS, redirecting the card holder to an authentication environment (an app) controlled by the card issuer.
Could you name me merchants that actually require just the number, and nothing else? I personally never really saw this.
And "normal" 3D-Secure is still "true SCA", SCA = Strong Customer Authentication, this just means, that there must be a strong authentication, but not that there is no name or expiry date required anymore.
bunq.me actually supports it, I just tested it and after I clicked on "pay" I had to confirm the payment in my banking app via Face ID, but your bank has to support 3DS for this to work.
If there is actually a new system, where you don't need anything but the card number, it might be still something worth looking into, but it's not a requirement for a strong customer authentication (it's more a privacy thing), normal 3DS with a good implementation by your bank should be good enough.