I'm not a fan of word extension, sometimes its easy to guess, and when you make it very difficult, you put yourself at great risk of forgetting the format one day and losing your coins too. When choosing safety measures, make sure you don't end up shooting yourself in the foot.
If you extend your seed phrase with a passphrase, you are not expected to commit the passphrase to memory, just the way you are not going to commit your seed phrase to memory, your passphrase should be backed up on paper, but in a different place from your seed phrase. Extending your seed phrase with a passphrase is a good layer of security and it can also be used for plausible deniability, so i recommend.