@OP my 4 year old is going through a defiant and highly emotional phase right now too. It's exhausting. I'm not pregnant and we haven't moved. But the posters mentioning anxiety helped a penny drop for me (thank you PPs).
It wasn't that long ago that our kids were taken out of school overnight, for months, with no idea of when things might return to a normal rhythm. And us adults were speaking about people dying and possibly worrying about dying ourselves, or at least getting badly sick. The pandemic has forced a huge upheaval and uncertainty on all of our children, and you've got additional (but totally normal) upheavals and uncertainties with a new house and a new baby on the way.
Is it possible she's worried your DH will love the new baby, his own baby, more than he loves her? Especially if she has a tentative relationship with her own dad? Is she weighing up his own happy reactions to a new baby and wondering about how her own dad was with her? I.e., "why did he go away? Was I not good enough to love? Is the new baby better than I am? If that's true, is Mummy going to go away from me too?" She may not be mature enough yet to express that kind of worry very well, but it might be worth addressing directly.
As far as "how strict":
If it's PJs and underpants that she's hoiking up, but it's only happening inside your house, I'd either let that slide and ignore it, or take the PJ shorts away. It's one thing if it's happening out in public. In the house, you and DP need to figure out together whether that's an argument you still want to feed into.
Meals are tough. DH and I don't want to cook five different meals, and I don't want to eat nothing but beige either. I've had the most success with meals that offer lots of choices for everyone at the table. Mexican, Greek, Italian, roast dinners. Sometimes, the best I can settle for with 4 year old DD is a few bites of rice, a tortilla wrap, and some cucumber slices at dinner time. But at least she's smelling what the rest of us will eat, and seeing our enthusiasm about the meals.
There is a lot of evidence now suggesting that junk foods and ultra-processed foods (most of the packaged stuff in the middle of the store) are highly addictive and can alter the way your body responds to food. You might be better off removing it entirely from the house (as far as your DD knows) and insisting on stuff like fresh fruit or a piece of toast for her snack. I have resorted to eating chocolate in my car alone before, when I've been desperate for it; otherwise, I'll get a meltdown from my girls about them wanting some.
I am a bit strict about TV and iPad content. More lax about the quantity of screen time than some of the mums I know, but I will log into our Netflix account through the PC, set age ratings on my DDs profiles, and blacklist entire TV programmes and films if I don't like the values they promote. If bad behavior escalates, I will cut off screen time completely for a while.