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.
How unique do passwords have to be in order to be considered safe? If they follow a pattern are they still safe or do these bots try alterations to the leaked passwords as well?
Like if your password to Reddit was reddit1234 and your password to Google was google1234, if the Reddit password leaked is your Google one still okay?
Probably not if it's a human but bots shouldn't be able to figure that out ya?
They are completely random strings for each site, so having one will not help crack the other.
But if people pick their own passwords, it tends to be some word like you wrote, and then a hacker could try and crack the others by guessing similar words.