Hi, we ran into this problem too. We didn't want any random posts showing up on certain posts so we did the fix you suggested on a different support thread here to take it back to the old days and just use tags for relevance so we would have complete control of related posts showing up.
So, this leaves us with a blank "Read More" section at the bottom of posts with no tags. For this reason, it would be nice to have a check bo under settings that you can hit when "If no related posts, hide title "Read More"" or something of that nature.
And also check boxes where you can pick exactly what it uses for relevance - tags, categories, or random, etc would be nice! Thanks.