(This is my first post here so please be lenient :) )
Background: I am a pianist and recently, I have become a [classical] singer.
Recall the Henle 'Levels of Difficulty' rankings for piano pieces, found here. I am looking for something similar for (classical) songs.
(Please try to keep discussion away from 'music cannot be ranked,' for yes, I know this fully well. I am just looking for a relatively general guide to repertoire difficulty.)