When considering the use of BIP85 child seeds as passphrases, or any other deterministic approach, it's important to be aware that an attacker could potentially brute-force the second secret (the passphrase) from the first secret (the BIP39 mnemonic).
The attacker would have to brute force a combination of secrets: the child seed used as a seed, and the child seed used as a passphrase.
To do this, the attacker would have to have access to the parent seed and would have to know the person is using BIP85 and know the wallet uses a passphrase. And the attacker would have to check all combinations of all possible indexes at all possible child seed lengths.
I'd say using a seed with a standard passphrase is only more secure than my approach if the passphrase is at least 6 words long - but that introduces risks such as typos and loss, not to mention the need for easy access to the passphrase every time the wallet is used, which means greater risk the passphrase will be found. My method eliminates the possibility of typos, it includes redundant backups, and since the version of my parent seed kept in my home is encrypted with a very strong key, there's no risk of it being accessed by a thief. If somebody broke into my safe deposit box at the bank, they'd find a metal backup of a seed, which means they'd find 24 words, but they'd have no way of knowing how they're used. If somebody breaks into the safe in my home they probably need an ambulance.
All of that being said... the most important part of my setup is security of the parent seed. As we know, all Bitcoin owners should back up their seed on paper and metal, secured in 2 locations only they have access to... but sadly, most Bitcoin owners don't do that. I definitely do.