Just use multiple. That's what I do. As backup you can also use OpenPGP cards which cost much less than a yubikey. Or a cheaper Fido2 token if you use Fido2 fo

Just use multiple. That's what I do. As backup you can also use OpenPGP cards which cost much less than a yubikey. Or a cheaper Fido2 token if you use Fido2 for SSH access (I don't yet but it's coming into vogue). An OpenPGP card will cost about a tenner, you'll need a card reader to use it but for backup purposes it's perfect.