I'm 51 and my weight is ok so my main aim is health and that includes maintaining or adding muscle but it's not clear to me why you ask, are carbs required for muscle growth?

For want of anything better, I am currently sort of following Dr Ede's Quiet Paleo with around 60 carbs (her bottom limit) to avoid keto which I tried, but it didn't have any noticeable difference. I also avoid spinach (oxalates) and minimise some high fermentables (Broc, cauli, apple, banana etc. per Dr Pimental's book) to control my nocturnal mouth reflux. I eat a wide range of animal products that I believe provide most of my nutrients.

To get back on topic, my question is, if I eat 60g carbs in one meal, how much fat can be included before insulin is unhealthily raised by the incretin effect. Conversely, if I eat a high fat meal, how many carbs can be included.

I doubt science has an answer yet, and like all things, it's probably complicated by individual variance, insulin resistance, type of carbs/fat etc.

Safest thing for now is to minimise fat or carb per meal.