> Storing a Wireguard key on a Smartcard isn't possible, because current cards do not support the key format and algorithms Wireguard uses. Only RSA and ECDSA on NIST curves are available on Smartcards.

There are programmable smartcards on which you can implement your own algorithm. ZeitControl sells cards you can program in a BASIC dialect: https://www.zeitcontrol.de/de/produkte/basiccard/basiccard-p...