I am not a doctor or even an expert on any of this, so…
The way I understand it, your body will be burning fat for the calories it needs to function after the insulin spike has subsided, so you will be losing fat. I would suggest getting a scale that measures body composition, including body fat. I have no idea how accurate they are, but mine seems at least consistent. I have a cheap wyze model, but there are tons of scales out there.
Drink lots of water anyway, it will help with the cravings if nothing else. If you are having a tough time with 15:9, you should just slowly increment to OMAD I would think. Your body is adjusting from eating all of the time, and being used to that, to eating less frequently. For some it may help to go slowly. 16:8 for a week. 18:6 for a week. 20:4… etc. But, if you think you can jump right in, give it a shot.
I would say that whatever you do, adjust some if you don’t like it. Some people will hate OMAD or 20:4, but thrive on 16:8. The goal, in my opinion, shouldn’t be to get to OMAD. The goal should be to adjust your lifestyle so that what you are doing is not a burden, but a healthy, easily maintainable solution to help your body, which will most likely include losing weight.
Good luck