Mbo,now I know where you are coming from--But you can decipher classical guitar music, which means that you're a better, more patient person than I am--Over the years, I have acquired quite a lot of fingerpicking technique, and the sort of people who are always trying to label other players often assume that I am a classical player--not true!!
I have never had the patience to work up a repertoire of classical pieces--the deciphering isn't even the hardest part, as you point out, the hardest part of playing guitar is getting those notes to speak with the precise duration that the composer intends--every time you play--
My first instruments were Trumpet, Cornet, and Flugelhorn, so I became pretty adept at reading classical orchestral parts-I also play "Composer" piano, which means I can play every phrase in a piece of music, just not together--
Sight singing is always unsatisfactory for the instrumentalist, because there is no mechanical component to producing the music, sight singing requires a leap of faith that the technician in us is never quite comfortable with--
For what it's worth, in the way of a tip, I was taught to worry about the reading the pitch first, and worrying about the note values later, but I do the opposite, I read through a piece to get a feel for the way the rhythmic cadences flow, then go back and figure out what the pitch range and direction of movement is for each phrase, then the intermediate pitches generally fall into place(even if they don't, I know what I have to work on)--