I'm not forgetting, I'm just ignorant!

Backlog and batch sign?

Doing signing on the card was the first place I looked. I found secp256k1 seemed poorly supported on yubi and other platforms. Maybe wrong.

Also, NFC cards are very cheap.

Thinking of ways to get 100's and 1,000's of users onboarded cheaply without them being able to stuff up and leak their keys.