At a loss.. 3.5 year old regression

My daughter is one smart cookie and I am pretty convinced she is "potty trained" in that she knows when she needs to go and what to do about it- but no no that does not mean she does go when she needs to go.

She has always struggled more with peeing than pooping. From the beginning she would come to us and say "I have a little poop in my body!" and we would take her to the potty. Peeing has been more of a challenge- it's very much a power struggle thing. She will be doing her dance that we know she needs to pee but will only go if we can think of a very creative & fun way to get her there.

But lately all attempts of using the potty has stopped. She will full on poop in her pants and full on pee (usually she will have just a tiny bit of pee in her undies which I think is just part of the process). We've tried sticker charts, we've tried having her wash out her own undies, we make her take a shower everytime after she poops her pants (which she haaaaates). Nothing is working. She has flat out told me she doesn't want to go potty because she has to stop playing. I try to explain to her that taking her to get cleaned up actually takes longer but no change.

She has a 5 month old baby brother and did start preschool 2 mornings a week in September- I know big changes can affect potty but I just don't know what to do about it. I try to stay calm and react as little as possible but MAN it's frustrating!

Please send all your ideas/things that worked for your stubborn, emotional, wonderful kiddos.