If that's the only objective then the solution already exists and there is no reason to re-invent the wheel. It is called the extra word or the passphrase. It is any phrase that you can add to your existing seed phrase to derive an entirely different set of keys. The seed is still the same and is the decoy itself. The only way to access the funds is to know that passphrase.
That passphrase can be the same password you used to encrypt in your algorithm (birthday, passport number, etc...).