I do these kind of optimizations for a living. Here is what I can recommend you prior to going after fancy plugins. You dont even need wp-rocket as it is just based on marketing crap. You have less than 1k QPS, that is practically nothing.
- switch to php 7.3, it has performance improvements over 7.0.
- add proper server caching (cache db queries with redis or memcached, add factcgi cache to nginx for frequently accessed pages for not logged in users and make the 404 page static, so it doesnt go trough the backend)
- protect wp-admin from brute force attacks. dont hide it, but rather leave it as is, but use some WAF solution to block with iptables people/bots who are trying to brute force it. this way brute force requests wont reach your web server. on a busy website this can add lots of load.
- check your database size and make sure it fits into innodb buffer pool. while doing that double check you have mariadb config suitable for your server.
- next step would be to find where is your bottleneck and plan next steps from there. usually for bigger websites you need to scale horizontally, not vertically (meaning you should use a few smaller servers, not one big server).
P.S. - this kind of work has nothing to do with the panel.
I hope my answer was helpful.
