After a weekend trip to Mexico with no DCFC's on the other side of the border (Baja), I got up close and personal with really slow charging. (Solar, 110v, shared).
On the way down, I got a lot of alerts along these lines: "Drive below 60 mile per hour to make it to your destination with 5%".
My suggestion is to make FSD listen to those alerts, and NOT drive over 60, or some other set speed required by low state of charge. I would dial back the maximum speed to 60, but every speed limit change, FSD threw that away and rocketed up to another speed, like 70mph. Let's just make FSD aware of impending doom in terms of running out of charge.
Ultimately, I made it to a destination charger, but at 0%.