While Dak has some leverage, he doesn't have all the leverage.
Granted, Dallas should not have let it get to this point. But my guess is the non-exclusive franchise tag puts Dak if done two consecutive years at 2/61 or in that ballpark. I am assuming that any longterm deal would average around 35 million a year or so. If Dak isn't willing to go at least 5 years on a long term deal, Dallas might be best off using the franchise for one year. A 3-4 year deal at 3/105 or 4/140 doesn't make much sense for Dallas. A third tag puts a 3 year deal at approximately the 3/105...add a little inflation due to the rising QB contracts and maybe it his 3/110. Either way, a three year deal doesn't make sense for Dallas. We are better off just tagging him.
So, the leverage isn't as much as you think. So, if even with the tag Dallas is looking at 3/110, then a four year deal at 35 AAV is to be the starting point.
Now, going the tag route, Dak could hold out and refuse to work out and such, but as a QB his value could be diminished the most my doing so. For Dak to get the payday at some point that he wants, whether this offseason or one in the future, he has to continue to improve. To me, Dak has to be a part of OTAs and meetings to do that, and if he takes a step back, it could cost him millions in a future contract, especially if he is playing year to year under the tag.
The largest piece of leverage Dak holds is that the Cowboys have no viable options other than to pay him. For this offseason, that is true to a degree, but those situations can change suddenly. If Dak does play on the tag this season, I believe the Cowboys need to look to find a QB in the draft, even if it is in the 4th round. Dak, himself was a fourth round player.
That being said, Dallas does have a final option and that is to allow Dak to become a FA and simply bid against the other 31 teams for his services. I don't expect that to happen this offseason, but it could in the future if Dallas believes Dak is being unreasonable.
I do think Dak has leverage, but the options of the tag give Dallas some too. I do believe deadlines make deals. There are two of these. The first is the March 10 deadline. If nothing is done by then, I expect the tag to happen, but also I expect a deal done by the July 15 deadline.