I'm sure this question will be downvoted, but...
Is Google Maps MarkerManager still "a thing"? On some of the Google documentation I see it mentioned, and in other parts of I do not.
All of the information I can find on this library seems very outdated. It also causes an error if this library is included on the page before the Google Maps API is loaded. Is there a newer recommended method for handling lots of markers the way this library does?
2 years Ago, when I was working on map based project, we used markerclusterer library. It might suit your needs. Here's a simple example
some more examples