According to over-the-CAP Dallas is $5.5 million over the CAP in 2023 based on a $225 million CAP number. This is determined by the top 51 salaries on the roster. This does not include any carryover from 2022. Maybe this is why the Cowboys are not using the CAP space they have now to sign players.
As for extending players coming off their rookie deals, I think this depends entirely on the willingness of the players to sign an extension without going into free agency. I don't think Diggs will sign an extension. I have a feeling when his deal is done he will be moving on from Dallas, perhaps to join his brother on another team. If I was an agent for such a player I would advise him to hold off on any news deals. The way contracts are going up, the chances are they will get more money later.