I'd suggest going for the fasted workout AND delaying eating again until you planned. Skip the post workout shake, that breaks your fast, no matter that your workout was 300 cal and your shake was 300 cal. It sets the digestive process in motion and the fast benefits end there.
Think of Grok out on a long day's hunt, no food for 15 hours, an aggressive sprint or bout of rock throwing trying to take down prey and he fails. Then at 20 hours, another all out effort to catch prey is thwarted, so again he rests after the effort. As he heads for camp, a rogue wild boar sends Grok up a tree to save his own skin, yowza, 3 hard efforts, lots of walking and moving about, maybe a nap here or there, and over 24 hours with no food. Plausible scenario, and likely something we humans evolved with for eon upon eon.
i don't think you are going to waste muscle by skipping your post workout shake! You'll likely get nothing but benefits from the fasted workout, followed by an extended fast! Go for it!