Unfortunately I never look at the calories on the label as I only look out for salt and proteins.
However, you can make the calculations based on the numbers I gave in my other comment - which are taken from Why Calories Don't Count from Dr Giles Yeo.
I think you still can't overcome 500 calories of deficit. If you ate pure protein you will still be in 150 calories of deficit using the most conservative numbers (30% overhead to process protein).
But this is assuming you calculate your daily calorie burn with one diet that is pure protein and then convert to one that has 100% calorie availability, you still can't overcome 500 calories of deficit on paper!
Realistically you would change diets, then find your baseline based on the new diet, and then calculate a calorie deficit from there. In that case you would be very close to a 500 calorie deficit, and you would unavoidably lose about 1lb of fat per week. A 500 calorie deficit sucks but you will adapt to it in a few months if you stick to it. I've managed to lose over 50lbs in the last year, probably close to 75lbs of fat mass. (I did use a GLP1, dieting is hard)
Our metabolism slows down primarily because it takes more energy to keep a fat person alive than it does to keep a healthy person alive. After you lose 20% or more of your body weight you might start to notice this, it isn't a real concern. The effect is negligible and again notice that people taking drugs thet let them comply with a calorie deficit diet don't have to resort to excuses like this to explain a failure to lose weight.
The cause of a failure to lose weight with moderate calorie restriction is always a failure to maintain the calorie restriction aka cheating on the diet (because it is incredibly hard to do when you aren't used to it).
I think you know more than me on the subject. I have never cared about calories in my life and never really cared about my weight.
I lived abroad for a year and, when coming back home, started eating healthily (i.e. more greens and less reds, avoiding added salt). In about 3 months, I managed to reduce my weight by 10% - about 6kg. Although I have to admit, if anything, I wanted to keep those kilograms because it was helping me for weight-training lol. My objective was never to lose weight.
However, you can make the calculations based on the numbers I gave in my other comment - which are taken from Why Calories Don't Count from Dr Giles Yeo.