Driving GA to CO: Only have State Farm Liability. What’s the cheapest way to insure the rental car?

I’m planning a long road trip from Georgia to Colorado and will be renting a car (likely from Budget or Enterprise). I need some advice on the most cost-effective way to make sure I'm fully covered without getting ripped off at the counter.

The Situation:

• Primary Insurance: I have State Farm, but it’s Liability-only. I do not carry Collision or Comprehensive on my personal vehicle.
• The Card: I have a Capital One Quicksilver, but from what I can tell, my version of the card does not include the rental car insurance benefit.
• The Risk: Since I don’t have collision coverage anywhere, I’m 100% on the hook for the value of the car if I hit a deer, get hailed on in the mountains, or get into an accident.

My Questions:

1. Is it cheaper to call my State Farm agent and temporarily add Collision/Comprehensive to my personal policy for 10 days, or should I just buy the rental company's Damage Waiver (LDW/CDW)?
2. If I stick with my Liability-only plan, is the rental company’s "Supplemental Liability" (SLP) worth the extra $15/day for a cross-country trip, or is my personal liability enough?
3. Are there any reputable third-party rental insurers (like Allianz or Bonzah) that are actually reliable if you have a claim, or are they a nightmare to deal with?

I'm trying to avoid the ~$40/day "full protection" package at the counter if there's a smarter way to do this. Thanks for the help!