Using Question2answer for high traffic sites? Share your experiences

Question

I am just wondering if anyone has pushed the question2answer framework to a high load, let's say 200k+ users per day.

Found only those posts:

http://www.question2answer.org/qa/7296/cache-for-high-traffic-sites
http://www.question2answer.org/qa/62295/performance-for-multiple-sites-using-the-same-database

Found later on:
http://question2answer.org/qa/645/is-there-a-big-website-using-q2a
http://www.question2answer.org/qa/50292/q2a-with-few-and-huge-traffic
http://www.question2answer.org/qa/6127/q2a-is-slow-do-you-agree

I wonder if q2a with 1+ mio posts is still performant enough.

For my current site: ~25k visitors per day, ~250 visitors/minute (in peaks), 500k posts

Server: Digital Ocean, 4 GB RAM, 2 vCPUs, PHP 7.2

Processes:
mysqld 86 %
php-fpm 11 %
 

Notes:

I guess an HTML cache (not the built-in query cache) would greatly help! – There is a plugin from 2013 (not tested yet): http://question2answer.org/qa/2865/is-caching-possible-in-q2a → https://github.com/bndr/q2a-caching → Found this now: http://www.question2answer.org/qa/46811/q2a-caching-branch

Thanks for any feedback, ideas, and experiences.