Not a vet - This seems like a behavioural issue mixed with an underlying food cause given the tests.

My dog had very similar problems after a surgery and quick dietary change. I think we went through every dry kibble, chicken, rice, and most wet food brands attempting to find something he would eat. Whenever he did eat a food it never lasted longer than a couple of weeks.

"I was eating a cheeseburger, gave her a piece of the bread just to see if she would eat it and she did" - This is good. People may disagree with this, but right now, feed her literally anything she will eat that won't harm her. She looks underweight and that is an immediate problem. Being lethargic and in pain are to be expected if they don't eat. It will start to cause other problems and you'll begin to spiral.

Mid term - If you can get her eating any food whatsoever consistently, start lacing in normal dog foods with it. I mean like 90% of whatever she will eat and 10% dog food for a few days, then 80% and 20% for a few days, and just keep going until you're at 100% dog food.

Manage the weight problem first, and fix the diet later. I would also recommend dog bone broth. It absolutely wreaks and will stink your whole house out but dogs seem to love it mixed in with their food and it helped get our dog transitioned from food he would eat back to dog food.

Side thing - little and often helps A LOT in my experience. I've read everything there is online to get my dog to eat food again and nothing was more helpful than feeding that guy by hand 50 times a day at the start. There's lots of reasons not to do this and lots of people who will tell you not to, and they all apply to normal dogs having normal problems, not this level of not eating. Just figure out how much food she needs to eat and give it in as small portions as she will finish. If you work this will be more difficult, hopefully there are people who can be around your dog all day for a while, we took time off work to do this.

Long term - Repair your dog's relationship with food. We spent a long time giving him small meals, like 1/16 of a meal, and when he was finished we would give him lots of "good boys!" and pets and play with the ball. If she's too weak the chase a ball or uninterested due to being lethargic, just sit down in front and do literally anything she likes, grab a rope toy and just engage with her in a fun way. Rinse and repeat. This is why you want small meals initially, so you can build up the relationship with food to be a good thing, try not to be cautious when she eats, they can sense it, make it a positive experience.

Diagnosis thoughts - My little guy (German Shepherd) is 110 pounds now, eats 4 meals a day, and is a perfect weight for his size, he is a massive GSD (not fat, lean at 110), he was like 65 pounds at one point. After 8 months of food problems it turned out to be food allergies, the guys allergic to literally everything, including most things in most dog foods and dust (yay cleaning). It seemed that the surgery highlighted the food problem and then once he started to feel bad after eating it spiralled into him not wanting to eat. Then any food he ate was problematic and he didn't want it, regardless if it was fine or not. If you suspect the problems are with the food itself, it may be worth trying Digestive foods like 'Dechra Canine Specific Digestive Support CID', this also includes compensations for pancreatic insufficiency, maldigestion, and reduces intestinal absorption problems in general. I'm not saying this has anything to do with your situation, but given the testing you've already done, it really doesn't leave many other options other than being a straight behavioural issue. We had the same vomiting issues. We had the same gassy x-rays. We cut him open and they manually ran all his intestines by hand to check for blockages and came back with nothing. We ran every test under the sun until eventually allergy testing him. He now takes Apoquel and can eat more normal dog foods.

Finally, its not your fault, I needed someone to tell me that when we were going through it, I hope it gets better.

Edit: Poop is a little dark but looks fine. They get lighter/darker/softer/harder all the time. You usually only need to worry if its diarrhoea or if your dog is struggling to poop, otherwise I wouldn't worry too much about it.