you are required to be able to power "expected" loads for automatic backup power sources. which means any thing that can switch on automatically, like refrigerator, heating/cooling, hot water, well pump... which means if you can't handle it in need to be on a non backup panel.
Correct, you are required to be able to start any of the loads on the backup system, but not required to start them all at once. This where the education comes into play.
Since the AHJ aren't requiring load calculations that prove the ESS can support all the loads its up to the installer to properly educate the customer and lead them to the best solution for the customers expectations.
Taking the power away from the customer is silly. No reason to add a whole extra panel for non backup loads with a customer who understands the limitations and either turns off his HVAC themselves or has a dead battery in 3 hours because there is no sun to recharge it. Instead allow them to put the car charger on the backup system so you have some place to pass all the PV generation once the stationary batteries are full. Leave the HVAC on backup so that when the summer heat is really pumping, perhaps the PV plus PW can run things quite well when irradiance is high, even for several hours depending on the time of year and energy resources compared to other useage.
I prefer to design where you don't "protect the customer from themselves" and instead educate them about the pros and cons of the choices you think are best with what you can provide at a reasonable cost. Just because throwing the car charger, HVAC and oven all on at once could overload a 3 Powerwall system, doesn't mean you can't still design it that way, just educate the customer: don't use everything at once.
Powerwalls are not damaged by overloading them, they have an internal load management system that protects them so there is no safety hazard to putting more on the load side that they could handle at once, as long as any single thing on the load side could be started.