Candida or bacterial overgrowth of the small intestine can cause everything you have.
The mucus build up is a way your body detoxes.
I have this bacterial overgrowth of the small intestine and I would choke on my own mucus. I started taking NAC 600 twice daily on an empty stomach for this. It helps tremendously.
I have or have had at one time many of your other symptoms. To get rid of these you need to start on a candida diet, detox, take anti-fungal medications and take a supplement to help heal your colon. The cause of you symptoms are from the bacteria feeding on carbohydrates. This causes the bacteria to release toxins. Thus causing excessive mucus, diarrhea, asthma, allergies, rashes, acne, muscle pain, feet and hand pain, and so on. I have much information on this condition on my profile page if your interested. Hope this helps.
I have mucus build up and have to constantly clear my throat when my vitamin D levels are low. If you haven't had your vitamin D levels checked then I highly recommend doing so. Optimal range is 40 - 60 ng/mL (100 - 150 nmol/L).
I too have hypothyroidism (Hashimoto's thyroiditis) and also have autoimmune pernicious anaemia (which is why I have chronic gastritis although I can't feel any pain). But gastritis can be due to low stomach acid (untreated or undertreated hypothyroidism is a cause of low stomach acid btw).
Your symptoms are all possible symptoms of untreated or undertreated hypothyroidism. Just to add, the top three low in the range or deficient nutrients with hypothyroidism are vitamin D, iron (ferritin) and vitamin B12.
Excerpt from Dr Mercola's article: Optimum Diagnosis and Treatment of Hypothyroidism With Free T3 and Free T4 Levels...
"In order to optimize the hormone replacement, the Free T3 and Free T4 should be above the median but below the upper end of the laboratory normal reference range. The goal for healthy young adults would be to have numbers close to the upper part of the range, and for cardiace and/or elderly patients, the numbers should be in the middle of its range."