Desperate need of advice for Alpha cat attacking new members of the house

I want to start off and say I have worked with animals for about 8 years, but I have never experienced a cat acting like this. I’m at a total loss for what to do, and I’m desperate for help because we are genuinely at our wit’s end.
My girlfriend has three cats. We recently moved in together a few months back. I added a dog to the mix. One of her cats, we’ll call her Feisty, HATES my dog. And I mean DESPISES.

We slowly introduced them, yet everything we tried was a bad experience. Before I officially moved in, I brought my dog for a few hours multiple times out of the week for slow introduction. We’d keep her in one of the rooms with the door closed so they could smell each other’s scents through the door. Feisty was NOT having it. She would come up to the door to smell and would immediately puff up and start hissing and growling. We tried giving them treats at distance from each other, she would eat the treat then immediately go to hiss and growl at the dog. We tried this multiple times a day to get them to associate each other with treats. Didn’t work. Ultimately I had to move in, and it’s been awful. Feisty will hiss and growl anytime my dog gets near. Feisty has attacked her on numerous occasions.
We have tried everything from timeouts, nose boops with a “no”, and to the point where we’ve had to scruff her and roll her on her back to try to get her to submit. She is out of control. She never attacks any of us, but when she attacks the dog she’ll try to attack anyone and everything near her as well.
We think we’ve pinpointed why she is so aggressive towards the dog, but we’re not totally sure. My dog is super anxious and very submissive to other animals, she was a rescue I got when she was 9 months, and she has anxiety off the charts from what I assume was a result of her being abused as a puppy. We think Feisty doesn’t like how anxious she is.
- Also she’s fine when we’re all on the couch and Feisty is on our lap and dog could be inches away, and she doesn’t care

Fast forward, we were put in a position where we had to take in a stray we found (very long story). But we did slow introductions for a week, all the other animals are fine with the stray, Feisty is not. She will scream like I’ve never heard in my life at this stray and try to attack. Stray is mostly unfazed by it surprisingly and doesn’t care for her or let her boss him around, half the time he seems completely unaware she’s even hissing at him. Im sure her problem with him is to try and knock him off the pecking order and figure out their place in the “pack.” But it’s driving US nuts. Because again, when she gets in the mood, she goes to attack at anything and everything near her.

Nothing is working, and we seriously have no idea what we can do. I’ve gotten to the point I’ve considered medication, but I also don’t like throwing meds at just any problem. She is stuck in this alpha, queen of the house mentality. And I don’t think meds would really fix anything other than slap a bandaid on the problem. But if meds are necessary, we’ll do them. I’d donate my left leg to get this situated at this rate.

TLDR: Alpha cat is having issues with new animals in the house and is attacking anyone and everything. Nothing is working.

If anyone has any advice, please help!!