The way I see the MM hire is that prior to his end in GB he was very similar to Payton (Almost identical records) consistent 10-12 win guy that gave you a legitimate shot at going all the way every year.
Being one of the more notable WCO's in the NFL, there should be alot he can add to Dak's game as a QB. You saw alot of improvement in Daks placement last year. Especially deep. McCarthy should be able to tighten up his short range timing based passing up alot. When it seemed like Dak had placement issues, it looked like it was in the 10-15 yard range. McCarthy will fix that
Dak is simply progressing like any good young QB. Moore brought the deep ball and some aggressiveness to the offense. Dak thrives in a wide open, hurry up, spread offense. Ironically, that is what McCarthy had in GB.
What MM will do is tailor the offense to suit Dak completely. It will be designed FOR Dak and his skill set.
My biggest issue with Dak is his movement within the pocket or lack there of. All the elite QB's shift around in the pocket and buy time by stepping up in the pocket and avoiding the rush. Dak has a weakness here. That is what I would focus on with Dak. And the added benefit to Dak stepping up into the pocket will be that he can then take off when things break down and pick up more yardage with his legs.
Watching Mahones, Rogers, Wilson, those guys are soo good at doing that. They actually set up blocks with the way they move around in the pocket. Amazing to watch.