Sorry to hear you're feeling so bad. Your story is familiar to all of us. Although our symptoms may vary, we all understand the frustration of feeling impossibly ill in the face of a Doctor who insists we are fine. It's as though they have deliberately chosen tests designed NOT to find the source of the problem.
I recognize many of your symptoms, and you mention all of the correct suspects. However, I think this can all be explained by a Candida/Sibo combination.
Also, it would be helpful to know what you have tried up to this point, and if you are trying anything now.
Has anything helped? What kinds of foods are you eating that make you feel better? Is it Sugar/Carbs?
Some ideas that occurred to me as I read your post:
1. Malabsorption of Vitamins and Minerals. You may feel better while you're eating because you are absorbing nutrients as you eat. As the food makes its way into your small bowel, you start to feel ill again because you are no longer absorbing the nutrients from the food. Candida Plaques, and Biofilms formed by overgrown Bacteria in the small bowel can block vitamin/nutrient absorption and cause this symptom.
2. Ketosis. It sounds like your body is going in and out of Ketosis in an effort to maintain glycogen levels. When your body transitions into Ketosis you can experience both a Metallic taste, and a Sweet taste. It can also cause abdominal pain. This supports the idea that your body is struggling for nourishment.
3. Autointoxication. Large populations of Yeast and overgrown bacteria in the small bowel produce powerful toxins that overload your body's ability to detoxify. Acetaldehye, Formaldehyde, Ethanol, Ammonia, Hydrogen Sulfide are variously capable of causing the seemingly unrelated symptoms you describe. Especially the mental effects, brain fog, eye floaters, mood disturbances, feeling in a altered state.
4. D-Lactate. This is a slowly metabolized Sugar produced by Lactobacilli. When these Bacteria overgrow, they produce too much D-Lactate for your body to handle, and one of the symptoms it causes is difficulty finding words.
5. Hydrogen Sulfide. This is a Bacterial toxin from microbes that belong in your lower bowel. In the upper bowel, it leaks out, circulates and attacks your Mitochondria directly. This causes fatigue at the cellular level, and makes you feel exhausted.
6. B-12 Malabsorption. B-12 from food sources is absorbed in the Ileum only. If overgrowing yeast/bacteria are inhibiting absorption, B-12 Deficiency results. This can contribute to exhaustion, poor concentration, and mood disturbance, and nervous system disturbances.
7. Meniere's Disease. This also might explain the Tinnitus and Dizziness. Meniere's is idiopathic, but possibly related to Candida/Sibo via Hypothyroidism, Autoimmunity, and poor Lymph Circulation/Function.
I think that it helps to view this as a "syndrome" without a single specific cause, but rather with multiple, overlapping causes. I've touched on a few, but there are likely many more. The unbalanced conditions that allow a single Pathogen into the body tend to allow others to follow.
I also think that you need to take action and get yourself to a Naturopath. A Naturopath will take you seriously and can influence your MD to dispense the right medications. I think this is fixable, but you need to get started right away.