i have very high cortisol too. I speculate the solution involves stringing together a few weeks or even months of 9-10 hrs sleep per night. I have no idea when I may be able to try this out. If you ever find something that works for you, let me know!
Prolonged fasting on day 3 onward could decrease cortisol (or actually cortisol won’t get increased after being exposed to stress) but intermittent fasting usually increases Cortisol.
Check
https://academic.oup.com/jcem/article/82/4/1101/2866203
I don’t think fasting can fix systematic Cortisol issues. I.e you might get back to previous state after breaking your fast.
You can try a fast mimicking diet. Look it up on the web. You can buy expensive kits or just make your own with the general guidelines.
It makes your body think it’s fasting but you eat a very low calorie vegan diet that is enough to Jew you feeling full.