Yeah, it has been proven that lowering calories lowers your metabolism. However, with fasting your calorie burn actually slightly increases. This is why OMAD, intermittent fasting and other forms of more extended fasting are all the rage. The Keto diet where you attempt to stay in a state of Ketosis (fat burning while interspersing it with fasting is a good bet for pre diabetic / diabetic). However, Keto focuses on fat in the diet with moderate protein and it's difficult to even get the good carbs right and some have to go as low as 10 net carbs). Some modify Keto to as many greens as you want.
It's a whole thing... Also, if you struggle to add the weight it may be useful to hire a personal trainer. I had to do that after my accident. I would like to do it again because I really built so much muscle in my upper body anyways it really paid off for the long run and I'd like to get another shot of That.
You can get a Dexascan that will show your muscle vs fat (vs bone and water, etc.). I would recommend you do a before and after at least ( a time or two).
All stuff from following these 'forums' and my own reading and practice for years. I know how it works; it's just hard to implement on yourself.