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!!!
400 sites are a lot, I do not use that much, but I just let pw manager deal with passwords.

However I have some sites that I use frequently and don't want to login everytime in the password manager. For those sites I use a simple method to create easily remember passwords.

It is actually two methods.
I create a whole sentence and the first word usually the name of the site, a funny sentence coming from the function of the site or the title of the site. A whole sentence including some numbers or signs no way to crack, except brute force.

The other method is take a poet and the first letter of every word in a verse gives you a nice hash looking-like password. you can tune it up with numbers or signs.