Hello,

i´m fairly new into Bitcoin but have now a hardware wallet with a 24-word seed. Seed is stored securely. Now i´d like to add an additional passphrase to my seed and move my funds from the seed-only account to the seed-passphrase account.

Now to the passphrase: Is six english words with 4 characters chosen from an English dictionary secure enough for the passphrase?
I thought about 4-characters words because i want the passphrase not only to write down but to use a Cryptosteel/Billfodl type of metal device to store it. With those devices i could put six words in a row and leave the other rows empty.
 
Or should i create 6 random words with a BIP39 mnemonic converter offline and use those as a passphrase? Then i would have to engrave them into metal which i think is more complicated than using a Cryptosteel device. Passphrase will be stored in a different place as the 24-word seed.

Thank you in advance for your advice!