I think the core of the question is: "What kind of insurance is reasonable for this path in the Winter in Iceland?".
Roxy seems didn't understand the differences among CDW, SCDW, Gravel Protection, and Sand and Ash Protection, which puzzled me for a long time before as well.
Firstly, CDW stands for Collision Damage Waiver. This type of insurance pays for damage to your hired car but it usually excludes windscreens, tyres, undercarriage, replacement locks, replacement keys, and towing charges. But CDW often comes with an excess, which means you have to pay for the excess amount when you make a claim.
Secondly, Super CDW brings the excess to 0. In this case, you don't need to worry about paying extra money.
Thirdly, the Sand and Ash Protection covers your rental car from the possibility of being bombarded by some leftover volcanic ash or sand from many of Iceland's vast sand fields.
And the Gravel Protection usually covers damages from gravel roads to the body of the car, the headlights and the front windscreen.
Icelandic insurance companies do not cover this type of damage in their CDW, and as such, this specific waiver was created to cover it.
Get back to the core of your question, the answer is Super CDW plus Gravel Protection and Sand and Ash Protection, no matter the car is expensive or not! Because repairing even minor damages on a rental car would be much more expensive than buying the insurance!