When I researched this two years ago the free Chase Sapphire/Chase United cards (same policy) offered better coverage than the Amex $20 policy (cheaper in CA due to state government regulations).
Liability is always included in a rental (unaware of any exceptions) but usually there is a possibility to buy up to higher coverage. I recall a few years ago on an Avis Norway rental that for the same rate, EU residents were automatically given high liability coverage. Non-EU residents were given about 1/3 the liability coverage, but were offered the opportunity to pay an extra $10/day to get the higher liability coverage EU residents were offered free. (I chose not to buy up.)
Generally when outside of your home country any talk about primary/secondary coverage is a waste of time. The only exception is when you are required to buy car insurance by local regulations (Italy, New Zealand, some others) and there is no way to waive it, and you also get coverage from your credit card. In that case the rental insurance is primary and your credit card picks up what the rental insurance doesn’t, the deductible and sometimes things the car rental insurance might not cover like glass.
Last comment: AFAIK only the Amex premium policy offers health insurance for you and your passengers covering medical treatment of crash injuries. It also covers theft of property from the vehicle.