If your password is unencrypted or poorly encrypted, having a random string vs custom password makes no difference. The whole point of unique and strong password is so that a poorly encrypted service does not compromise your properly encrypted service. The scenario where my password is unencrypted is irrelevant, because only the salted hashed password matters. And because of the hash, leaking unencrypted passwords does not make the hashed ones easier to guess.
The whole issue with a manager isn’t that its bad, its that it puts everything under the one basket, even if its a hella strong basket. If you want to change my mind, you need to show the pros outweigh the cons. Straight up assuming that not using a manager somehow means anytime I have my password compromised equals everything else is compromised is not convincing, its circular reasoning.
Ignoring the fact that I’m explaining how hash works and not giving advice, if we want to be technical then yes only a slight change does make targeted attack easier. At that point password will only provide so much security, if you want to truely be safe, grade separate your username and email.