> we're still on MySQL, not MariaDB, and probably won't ever get actual support for this issue. PHP 7.0, ubuntu 16.04 and 18.04, doesn't matter. wikimedia left us in the dirt
MySQL is still supported, and what you are describing does not sound like a db issue, so I doubt its related to your use of mysql.
You should check your php error log of course to see if there are any errors in it. You should also run the runJobs.php maintenance script, and see what it outputs, if there is an error it will likely output what it is
> truncating the jobs table doesn't solve the problem
Deleting the jobs table definitely is not going to solve the problem. It might create the problem, but its not going to solve it.
> $wgRunJobsAsync to false doesn't solve the problem
This can sometimes help, depending on what the root cause is. Even if it did help, it won't help immediately unless you clear the backlog by running runJobs.php.
As something to test, try putting the following in the bottom of LocalSettings.php
$wgJobRunRate = 25;
$wgRunJobsAsync = false;
This will slow your wiki down quite a bit, so its more just something to try in order to debug. It also may not take affect immediately, so you might have to wait a couple edits before it works (Say about 35 edits).