The goal should be to strengthen your metabolism. At my leanest 8% at 165lbs I was eating 3600 calories regularly. It took me a year getting my metabolism there. Adapting takes time and fasting shouldn't slow your metabolism. Under feeding will. Going with The warrior fast let's say is a 20:4 fast that promotes berries and veggies throughout the day under 500cals then a huge meal before bed.
Metabolic health is based on regular fuel and if you're so lean that you think you'll under eat then you may need to not fast. Or just fast and be okay losing some protein from the body and enjoy the health benefits. Fasting isn't impossible when lean, it just hard.