Use the "passwords" feature to check if one of yours is compromised. If it shows up, never ever reuse those credentials. They'll be baked into thousands of botnets etc. and be forevermore part of automated break-in attempts until one randomly succeeds.
I use a "password pattern", rather than remembering all the passwords, I just remember a rule I have for how passwords are done, there are some numbers and letters that change depending on what the service is so every password is unique and I can easily remember all of them as long as I remember the rules I put in place
So when someone figures out your rule he has all the passwords
What's more likely, a password manager gets a breach or someone targets only me and manages to find out multiple passwords across multiple services and cross compares them works out what the random numbers and letters mean...
No you are right, your method is stronger than using a password manager hahaha of course there will never be a targeted attack or anything like it