> Password manager might be just an algo inside of your head.

This is dead in the water as soon as you realize that a leaked and cracked password can reveal all of your other passwords. If you want to say that your algorithm can't be reverse-engineered I'd be more inclined to think that you are underestimating the people around you than that I am underestimating you.

Probably best to just consider that your not-randomly-generated password is insecure.