I might challenge your assessment here.
All players from Mahomes to the last player on the worst team's practice squad have shortcomings. It is the coaching staff who are tasked with putting the player in his best light, and protecting him from being in a position to fail because of these issues. Obviously there are some players who do not have the skill set to rise to the level of excellence. But that is not the case here.
This team's success starts and ends at the offensive line. No running lanes, and no pass protection in a consistent basis is why you see losses like the one this past Sunday.
I am not a stats guy, per se. I somewhat agree with your comment because I think stats can make Charles Manson look like Mother Teresa. But there is an aspect of the yardage Dak throws for and its concordance over time which indicates, with protection and a competent core of players at the skill positions, he can deliver.
If for no other reason, his rookie year indicates when you put the right guys around him, and give him protection, he can drive the ship.
I agree with you that the success of a team starts with the big boys up front. Offense and defense line.
I do disagree that our offense has a pass protection problem or as you wrote "no pass protection on a constant basis".
A big problem of our QB is that he is not able to use the protection of the OL to his advantage (he has no feel for the pocket). He also loses control quick when feeling the slightest form of pressure. That makes the OL look bad. But they are not. In fact they a a good one in pass protection.
Have a look at this list where you can compare sacks/hurries/knock downs/blitzes Dak has faced to other QBs:
https://www.fantasypros.com/nfl/advanced-stats-qb.php
You can see in most of the stats he is either at the bottom or in the midth of all QBs. Which means he faces way less pressure then the average QB in an NFL game.
An interessting column is the bitz column: compare him being blitzed to other (considered) good to great QBs. Se how often team are blitzing QBs who they consider being able to hurt them by passing the ball ? Dak is not being blitzed a lot. And this is not because he is so good against the blitz. You just dont blitz bad QBs. You take away the run fill up the secondary and dare them to beat you with the pass.
I agree with you that Dak has games (and a lot of them) where he has good looking stats. I think this is a combination of a lot of things: Playing against weak defenses (like the Eagles on sunday), having a great supporting ast over his whole career (great to good OL, great to good talent at the WR and RB position). IMO he also had very good offensive coordinators most of his career. Those knew his defencies and called pass plays that he is able to execute.
Stats have to be interpreted. Interpretation is the hard part. Indexes like QBR or else are simplifications that take a lot of important information out just to have a fast way to get a first look at a player. They are not useless, but they are far from being able to judge a player.
For instance all those QB indexes dont take into account what defense the QB played against. Well every Dak fan was so happy after sunday "Dak is back to his old self", "he played an awsome game" etc.... But against what defense ? I mean if he played against a high school defense and put up 500 yds and 6 TDs would you also say he played a great game ? Just because of those numbers ? Now is he a good QB because of that ?
Dak has a clean pocket a lot of times. He can do some damage when he has one, but that is no measurestick for a QB. Every NFL QB (well at least those who are medicore at least) will pick apart a defense when having a clean pocket and enough time. So this is not a good way to judge a QB. Its much more interessting and also much more important how he does under preassure and or when being blitzed.
Have a look at this listing:
https://www.pff.com/news/qbs-in-focus-pressure-and-the-blitz
Look for those QBs who are considered good or great ones. Rodgers, Manning for instances and where you find them in the "under pressure" table. Just a sidenote: look where you find Tony Romo and you will understand why some here dont consider him a good QB.
A last thing: I try not to judge a QB during his first year - well, i try to, unless he is such a joy to watch like Mahomes for instance. I usually will find my judgement during his 3rd and 4th year. During the decades (25+ years) i followed the NFL defense coordinators always take one year to account fro a new QB. There may be a lot of reasons but the most important is i guess that they need a lot of tape about an QB to find out about his strengths and weaknesses. Then in the offseason they try to find ways to force him into his weaknesses.
I agree that Dak had a good first year. Especially for the QB he was known for comming out of college. But you have to admit that he was surrounded by what was considered a SB ready team. All there was left (opinion of our FO and coaches) was a stud RB. And they drafted him in Elliot. Yes you still have to perform. And Dak did. But that was his first season and long ago. Since then he has shown against most good defenses he cant perform against them. He may have a drive here or there, but on a constant basis - no. And dont start with bringing up the TB win in the last years playoff. TB was the monkey we had to get off our back. TB wasnt a good team that year. They were a team with a losing record during the regular season that snuck into the playoffs but in all seriousness had nothing to do in there.