I'm using 5.7.29 myself. MySQL 5.7.x is going to be JUST FINE for any XF 2.1 installation. Users aren't going to notice any difference in database performance between 5.7 and 8.X. Much more performance will be noticed in the broadband connection than in the database. As said, 8.x is much newer than 5.7, and is going to contain more bugs. 5.7 is more tried and true.
I'm on Ubuntu 18.04.3, which is the latest LTS version of Ubuntu. When the next LTS version comes out in a few months (should be 20.04) I'll do a database upgrade. That will give things a bit more time to mature, and bugs to be killed.