Hi, I don't think you need to install a memory monitor. You can just get away with top / htop or what you just gave me. If that document are correct you are only using 29% of your RAM. So that means you can increase for example your PHP-FPM workers, if it's those who are dying. Only way to know are checking the logs.
Depending on your database size you can tune it so it reads everything off Memory instead.
200 people online (if you check your Google Analytics) stats, don't do 200/request per second. If you don't do AJAX request every second to your server. At the maximum ~25-50 request / second. And that's a high number. And some bursts for images/css/javascript for new users of course.
Are everything user specific content or are this content static for guest? If it's static I would cache it with Nginx.
If you do 20 tests, that is 1640 clicks every second. That's a high number, never seen that in real life. And you need much beefier hardware than you got.
