that's why it is NOT only about kcals intake

Certainly there is that other slice - 1% - but it's not just carb/insulin response. If you want to get into all those weeds about the human metabolism then you'd be looking into leptin, cortisol, ghrelin, thyroid hormone, and a bucketload of other factors too. So many, you could practically spend your entire career puzzling over them.

But while you focus on all that - you tend to forget about the 99% slice - cal deficit. That's why we tend to say that's all that matters for weight loss.

This 99% - 1% is not a right proportion at all.

You are perfectly right: it is not just insuline, hormons works in a wires and feedbacks. Nevertheless: if you have a look at lifestyle related disease prevalences, it is the insuline resistance/pre diabetes/ diabetes that plays the "99%" roles. Thereby, major role of insuline here it's clear.

You mentioned other hormones, cortisol and GH in particular.
I saw plenty of people eating on a dramatic (unhealthy) Kcal deficit. They lost weight as well. Mostly lean mass, very often with the skinny-fat body and still abdominal fat regardless a normal BMI.
Why if they hypocaloric? Because starvations leads to cortisol increase. Cortisol is diabetogen, which means insuline resistence again. Insuline resistence, which will kills your effort even if you eat only 300 kcal ...in carbohydrates.

Kcals deficit is necessary because of energy laws. Kcal deficit based on unbalanced/carbs rich plans is the worst than not dieting at all, since you'll lose, but NOT fat. It depends on what you want of course, but there's no point in a mere scale/number goal I think.