"Reading Defenses" is a pretty vague term. My opinion is that Dak is pretty good at pre snap reads. You draw up a play on a white board and he will walk you through it with ease. What defenses have started to do this year is adjust post snap, and Dak has struggled with his post snap reads. Where I get stumped in my watching Dak film is you have guys in Cooper, Lamb, even Schultz that are all excellent at creating space. You'll see these guys open on tape, but Dak misses his window. Not sure if thats an inability to read coverage, unable to feel pressure in the pocket, simple lack of confidence, or something else all together.
Total guess here, but it almost feels to me like Dak is reading too much instead of just simple find the open man, and get rid of the ball. That's honestly what made him so good early in his career. The guy just dropped back, found an open man, delivered the ball. Didnt rack up 400 yards, but moved the chains and limited turnovers.
The Cowboys seems to go out of there way (maybe to a fault) of giving Dak easy reads. It's why they often line the RB out wide then bring him in, limit pre-snap motion, etc. The offensive scheme seems to be overly simple, and to be honest i'm not sure if its limiting Dak or thats because of Dak.