> basically you want to have a small supply of starches to keep your skeletal muscle from breaking down to feed your brain

But starvation in a healthy person takes 10-40 days to kick in, with 30 days being way more common than 10. Before that, you're just ketogenic.

Everything I read, and my personal experience, leads me to believe that the only muscle that breaks down is that which is not used (which is compatible with your general description). As long as you keep exercising during the fasting days, there's no reason to resupply your glycogen stores. After about two days, your body will switch to ketones as a fuel source, which will likely alter your body odour and -- if you are not used to it, e.g. the first time you do it, is likely to give you headaches and other flu like symptoms for a couple of days. But no reason for starch intake.