Considering point 4. maybe this could answer your question? https://www.trainingpeaks.com/blog/the-peter-attia-approach-to-dieting-for-endurance-athletes-part-2/
You can wear a continuous-blood-glucose monitor to measure glucose levels (which should be directly proportional to insulin if you are not diabetic?) and based on that track your fat oxidation. If you are at the baseline levels of blood glucose during an exercise it means that your body is oxidizing fat instead. However, they seem to be incredibly expensive, but Interesting article nonetheless.