Question: I have lurked for a time on security websites and similar. I came to wonder, with the number of passwords we have (or websites that need a password) how to manage this.

Since I have well over 400 sites where a password is used, how can I keep track of all the passwords and avoid reuse?

I have a password manager but still have passwords that are used way too many times. Is the best solution to just make 400 different passwords and let the password manager deal with the issue and look them up every time I want to login? 

Or is reusing for minor accounts the best alternative in terms of usability?

Answer: 
Yes - let your password manager handle all of them.

400 sites is not a lot to have in your password manager - I have many more than that. Most I generated randomly. Some I created on other systems and then transferred them in. But most importantly, I don't worry what they are, because my password manager handles them.

Before I used a password manager, I tried patterns, but they are relatively easily guessed once a couple end up leaked and brute-forced, and yes, I have had the haveIbeenpwned alerts on occasion and had to change things. Trust me, doing that is much worse if you have used any pattern, as you have to change them all!!!
The short answer is yes, let the password manager handle it.

You should have your password manager generate a unique password for each site. Don't make them yourself and you won't have to worry about reuse. For sites you currently reuse on, incrementally replace them as you come across them. Many have tools to identify password reuse which can also help you.

Based on your comments, you seem hesitant to commit to all random passwords because you use several different devices. Many password managers offer syncing solutions that work very well. Both Android and iOS support filling in login forms with password manager apps. If manual entry is a sticking point, consider generating [pass*phrases*](https://www.eff.org/dice), which are easier to remember momentarily.