I have Hashimoto’s and Addison’s disease and haven’t had any issue, so I’m not sure how to help. Undiagnosed hypothyroidism is what causes the average 5 - 10 lbs of weight gain and most of that is due to water retention that untreated hypothyroidism causes. I have to supplement my thyroid hormones and my cortisol and aldosterone and haven’t had any challenges regarding the Hashimoto’s. There are no specific dietary plans for Hashimoto’s. What is known to help overall, is to lose weight. Fasting isn’t magic exactly- it just helps sticking to a calorie deficit easier. Do you know how many calories you need to eat to maintain and lose weight? Eating a calorie deficit is not scary and I doubt you’re going to get “scarily low”. At 5’2 and I’m assuming mostly sedentary, you’re going to have a (what will seem) low calorie amount regardless. You have excess body fat for your body to use as fuel though. You will be okay eating at a calorie deficit. I recommend investing in a food scale to accurately track your calories. Low carb/keto is also very helpful for health issues. I’ve found less instances of low blood sugar and other issues with low carb and my various hormonal/autoimmune diseases.
I have the same health issues as you, plus PCOS.
I’d suggest you stick with intermittent fasting, at least for a while. You can start with 12/12 for a few days and work your way up to OMAD. Then decide what works best with your schedule.
Keep track of calories - especially because you say you might have food addiction and eating out of boredom. You must be in a calorie deficit.
Use an app to track your fasts.
As for coping with hunger and feeling “sick”, your meals (at least the last one) should be low carb. Other than that, just power through and remind yourself why you’re doing it.