The passphrase is very important, because even if the generated seed has low entropy, I can increase the security of the mnemonic by extending it using passphrase.
For passphrase generation, I recommend using 6 random words or more. Random words of 6 words have, for example, the same entropy as a password like "#\KL`@cZ3W*-", I usually think that 8 or more is great.
If seeds are created with low entropy then bruteforcing would require less time and the passphrase may not be just enough to stop the attack unless we are talking about a long sentence used as the 13th word. But I don't recommend either of these practices, it's better use the wallet itself to create the valid phrase whether it's Bip39 or others and the extra word can be something that you can remember easily, so it's better to be 8 characters than 8 words. For passphrase generation, I recommend using 6 random words or more. Random words of 6 words have, for example, the same entropy as a password like "#\KL`@cZ3W*-", I usually think that 8 or more is great.
