Point of multisig isn't to increase cryptographic entropy to reduce brute force of a single device. It's to distribute signatures amongst multiple devices in different geolocations and under different custodians ideally. This makes it such that multiple different devices have to be compromised for your UTXOs to move. It's an inherently different threat model than just creating a longer passphrase. You can have a 48 word seed phrase as your passphrase and have identical vulnerability to a wrench attack than if it were just a 12 word seed.

A passphrase is not a multisignature. It's a single signature that's encoded by the concatenation of the seed phrase plus the passphrase hashed into whatever the encoding algorithm is.

Please please please do your research idk why you consistently make giant errors in your "advice".