car rental insurance coverage by CC

My friend and I are visiting Iceland from the U.S. in 2 weeks, we plan on drive the full circle in about 7 days, just paved roads, no offroading. Currently have an economy car on hold with Hertz, I have the Chase Sapphire Reserve card and friend has the Amex Platinum, both offers primary coverage for ollision damage (may need to pay a small fee to Amex but it's negligible). Is this coverage adequate for our one week in Iceland if we're both good drivers, do we still need to purchase additional insurance? If additional coverage is recommended, what should I buy and should I just accept whatever Hertz offers at the rental car counter? Or is there better coverage that I can purchase before my arrival? This would be our first time to Iceland and first time renting a car in Europe, thanks in advance.