I think based on the evidence it is mental. If you watch games from early in the 2021 season, you see Dak making quick decisions, hitting open receivers, squeezing some passes in to both Gallup and Cooper and spreading the ball around. Later in the season he is slow finding the open guy, often doesn't see the open guy, he holds the ball too long, throws from awkward positions, makes mental errors, like the pick 6 he threw against Washington with a 13 point lead and 4 minutes left in the game, and he decides to run when it is too late to escape the pocket. In 2019 we saw something similar. Early in the season he was throwing well, then late in the season is mechanics fell apart and he was throwing wildly.
Dak was never pinpoint accurate, but he has shown he can be accurate enough.
Dak might have lost a step from his injuries when he runs but his throwing problems are not physical.
IMO, the Cowboys should not rely on Dak to move the offense and win games by throwing the football 40 times a game. He can do it once in a while, but it seems like he has games late in the season where he does not see the field well and struggles to get the ball out of his hand. The Cowboys need more balance on offense. They don't need to run for 200 yards a game, but they need to run more than 20 times a game and throw less than 35.
Jerry made the same mistake with Dak he made with Zeke and DLaw. He gave them more money than they will ever justify on the field. But then made it worse by blaming Cooper and Collins for the Cowboys CAP woes.