Give Cersei months/years in the South where the winters are not as bad and where she's not actively fighting a war, and it's unlikely you would be able to just take it back, especially after inevitably heavy losses in the North.

There is no chance this war is going to take months or years.

Cersei has been racing the clock to solidify her position, given enough time she could hire every sellsword in the free cities, and even work to bring Dorne and the rest of the Reach to rights and have those remaining armies and resources brought back into her kingdom under loyal bannermen.

Why would anybody fight with Cersei against Danys dragons?

It's suicide. Jaime leaving her army - and saying it's suicide - surely proves that to any doubters.

A straight up truce is the best case scenario for Cersei.

The best case scenario for Cersei was killing her the second they landed in Westeros. But, failing that, threatening to kill her is the solution I'm proposing. And then if she does anything you've said (like try to win back the Reach/Dourne) just jet-pack back to Kings Landing in 24hours and burn her and all of her armies... problem solved.