You're probably right. The obsession with the Darth title does seem to start here, as I can't recall of any other Sith naming themselves Darth so and so before Revan. I would assume the Sith Emperor started the trend of having the elite being called Darth.
There is Darth Andeddu, but he seems to be an isolated case. There is a theory that Darth is comes from the Rakatan "darr tah" = "triumph over death". So it makes sense that the Andeddu who lived very long through essence transfer would choos it as personal title without making it a common title.