Check out these three posts about die-off:
Acupuncture is also a very effective way to detox and heal the body.
A few of your symptoms are not related to thyroid in my opinion. The hair and nails symptoms are related to minerals, or lack of them. Iodine is a mineral though…but you likely need many more. Most people are deficient in trace minerals so don’t freak out about this. I’d get some calcium, magnesium, zinc, and selenium for starters and see if this helps. Solgar makes a combo calcium/magnesium/zinc mineral vitamin and this can reduce costs. Selenium is sold by many brands. This also should help with constipation as well.
I hear you on the confusion of symptoms and I once had this as well. The most important thing you need to do is reduce your stress (right now and long term) and put faith that you will feel better in time. Time is the most important thing you need and it takes a long time to recover from candida and other health problems. You likely have multiple issues and this needs to be unravelled.
From my experience, I would eat rice and quinoa about 1x per week each max or less if possible. Buckwheat is a great replacement and is a prebiotic that feeds the beneficial bacteria in the gut. If I recall with my scant knowledge on SIBO, prebiotics and probiotics make it worse.
I would get tested for SIBO asap. Have you ever taken antibiotics?
If its not SIBO or candida, you could have parasites. Everyone has parasites but at different levels (same with candida). There is a wide variety of them and we simply could get them from the food we eat. Don’t worry though, they are killed the same way as candida and they also release die-off toxins.
Please list your current diet and the supplements you are taking.
I would try upping the amount of vitamin C you take and see if your die-off improves.