As you get stronger and put more and more plates on that DL you will feel it ore and more in the upper back, and will want and get a strong upper back from it. BUT its from the static hold you are doing. its not EVER going to be more of a workout for your upper back than it will the glutes, hams, low back, hips, hell even quads it just wont.
Yes its putting the WHOLE damn body under a hell of a load but it isnt going to magically replace stuff dedicated to the upper back. rows etc.
you could make some crazy hybrid of a DL and add a press to it and a sqaut. To make it work the upperback and everything more. LOL the bear comes close. You could row the bar hold stiff leg it up, press it over head, drop it to your shoulders, sqaut, re press it up and then do a slow negative of an upright row followed by pushups, there now you got a complete exercise LOL
Nice work on the adjustment to get pulling right keep pulling and do some heavy rowing, chins, etc to nail the upper back further.