also i checked, the check bytes of BIP-39 are a byte on the right hand side

firstly, if your hardware supports this, it can rule out any string that puts a 0 on the MSB on the left as being nostr key

secondly, the bip-39 check on the LSB is highly unlikely to match up, it is a 0.5% chance of a valid bip-39 key being imputed from a nostr secret key mnemonic, probably you can cut that in half even since the MSB has to be a 1