If you are carrying kit that is so heavy it will seriously inure you in a crash are you not better with a van. Built for carrying loads etc.
The point is, even quite light things can seriously injure you in even a relatively low speed crash if not secured properly.
You are quite right about securing loads properly though. The back seats of cars are designed to withstand high loads when upright.
In the UK the Highway Code rule 57 states
Rule 57
When in a vehicle make sure dogs or other animals are suitably restrained so they cannot distract you while you are driving or injure you, or themselves, if you stop quickly. A seat belt harness, pet carrier, dog cage or dog guard are ways of restraining animals in cars.
Sorry, but this is one of my hobbyhorses having seen the result of what can go wrong.