What is a good way to backup the TOTP codes/recovery codes? Another Flex? Or save the recovery codes in plain text elsewhere?
Another chip is a good option, personally I store the TOTP secrets (i.e. what is encoded in the QR code) plus the recovery codes in my KeePassXC database.
Print out the qe code pages to paper and put in a drawer offline.
I do have two apex chips for this purpose… righty and lefty