Rental Car Insurance.

I cannot answer this question but if you plan to drive a car in Germany ensure the validity of your driving license in Germany and also the likely need for an IDP to have valid driving permission [not license only] for Germany. Violations are expensive and in case of an accident a driver without valid permission loses the insurance coverage.

Is this still a good way to buy insurance coverage?

Did you make a claim with Amex previously? If not, it's unlikely that you can say with certainty that it was a good choice back then. A claim is where the rubber hits the road.

Answering this question would require a comparison of the current Amex plan details and its real-world results with those of the many other insurance plans that compete with Amex for your $... it's a very big ask. I suggest doing an online search for consumer research that may have been done by sources independent of the insurance industry.

I would be surprised if there is no better specialized online ad-hoc insurance offering but I do not know this market.

I am searching for and reading reviews of people who have filed claims using AmEx.