I’m always perplexed as to the term “cheat day” because I’m my mind who is the one actually being cheated on? My GP? My DSN? My F&F? - none of those only myself. I’m cheating on myself.
By all means have a day where you plan to eat something you normally wouldn’t, we all do it, but I just refer to them as part of my meal plan as in planned treats. But these days most of my planned treats are low carb anyway such as a more expensive steak than normal, low carb pizza made with a base bought on line, a very good cheese.
If you want to drop your Hba1c by 15 points in 3 months, that’s a pretty tall order then I personally wouldn’t be thinking about touching anything like pizza or fish & chips, or anything that would be considered a “cheat meal” at all, those type of things for me are a one off very once in a while, maybe once or twice a year if at all. Especially bring on steroids also.
You may find if you can reduce your carbs a good bit your craving for pizza & fish & chips will reduce too.