Just scratching the surface, it seems to make sense to conclude that Dallas goes as its defense goes. After all, from 2016-18 the Cowboys rank 4th in points allowed in the regular season, and only 12th in points scored. But that doesn't really tell the story. Offensive ball control is the big reason behind those rankings, as no defense faced fewer opponent's drives over those three years than Dallas. Actually the Cowboys only rank 14th in TD allowed per drive over that time -- the defense just hasn't had to face many drives, because the offense led the league with the longest average possession at 3 minutes even.
In 2018 especially, the team's plan was to play conservative for three quarters to protect its own defense, then let loose in the 4th, when the defense was out of gas. Dak's ability to flip a switch and become much more aggressive late in close games played a big part in this strategy, because it allowed the defense to go all out for three quarters.
Dal OPPONENT drives ending in TD
1st-3rd qtr 17.6% (4th)
4th qtr/OT 31.4% (28th)
4th qtr/OT, margin 8 pts or less
15+yd targets as % of all attempts
Prescott 26%
NFL avg 21%