Is your server on a VM where you can allocate more memory to it? If so, do so ... How much? So you can tweak DB config for the recommendations MySQLTuner suggest.
If not on a VM where you can allocate more memory nor tweak MySQL config for higher InnoDB buffer pools, then ...
you need to split your DB to another server dedicated to just the DB for the moodle. You can then tweak InnoDB buffer pools to handle the size of the DB. Most of the DB will be in memory of the dedicated server so you'll also gain in that on the code server no SWAP space is used. Bet the SWAP current setup is using is DB. DB's perform poorly when using SWAP.
Now that you know that thing you did causes issues, don't do it during prime time ... only in non-prime time.
That 'rule of thumb' you've quoted above is and has always been a 'ball park' recommendation but don't think it's ever been 'accurate'. But, moodle has been known to be a memory hog.
My 2 cents ... am sure others can jump in here and provide much better advice.
'SoS', Ken