Embrace the limits of portable kits:
Nobody can afford to stockpile enough stuff to prepare for an extended duration mass casualty incident, not even big, well-run hospitals: capital cost, product deterioration over time, storage space and reluctance to spend scarce resources militate against it. Everybody relies on the supply chain remaining intact.

First aid kits, ambulances, and emergency rooms are designed to provide care for the first hour or so of injury or illness. The rest of the plan is transport to a place where the patient can receive definitive care. If the transport piece or the definitive care piece are absent, no amount of first aid training, supplies or good will will save the patient needing more care.

If the trucks and trains stop running, we will be back to 19th century medicine pretty quickly. If you want to know what that will be like, volunteer for a medical mission trip to anywhere, or find someone who has been there.

I can do a dissertation on ether anesthesia, home-made IV fluids and epsom salts, but nobody really wants that...