Hi PC.
Hmmm. If I remember rightly, your ex mainly imposed discipline by instilling fear - especially in sensitive Ds1. Do you think that now he has been removed one step away from fear, he is either confused about what now constitutes discipline, or else that now he knows his Dad is at arms length he can 'take advantage' by playing you up? I have no doubt that he also loves his dad, and behaves well with him because he wants to make sure his dad will continue to see him. But he knows it a choice, to soeme extent, for his dad, whereas you are for keeps.
It could well be a mixture of 'celebrating' the end of fear, of nnnow not knowing what will replace fear of his dad as discipline, and playing you up - as well as reacting with upset and anger to changes.
He probably feels sorry for his dad, too. As far as your ds is concerned, you are the most important and wonderful peron on the planet - perhaps he feels sorry for your ex that he has been rejected by the most important person in the world.
But I guess however complex it all is, deciding on a firm and consistent approach to how you want to do discipline will do no harm. Pasta jar, or something.
And, as Dino says, let them let off steam and be creative with messy outdoor stuff when they can.
But mostly, just keep venting on MN - and make sure you save lots of treats for yourself - lovely bath stuff, etc. You really deserve some nice times!