Hey guys, I am an independent agent and represent most of the carriers that you have mentioned here including Hagerty, Progressive, American Mod and others. I would be happy to answer any of your questions if you want to PM me or call me. I am licensed in Texas but would be happy to answer your questions regardless of where you live.
At a high level, majority of what you guys are saying is correct regarding stated value versus agreed value (or in Hagerty terms, "Guaranteed Value"). You have to be careful with stated value with some carriers as the policy is actually cash value UP TO stated value. This is what can get you as they will appraise the cash value and only pay up to that amount not to exceed stated value.
With agreed value or guaranteed value, you will received that full amount in the event of a total loss, no questions asked. If your vehicle is stolen and recovered, you will not get total value since it was recovered. If it is not recovered, it is considered a total loss.
As stated, there are different requirements with agreed value policies which I'd be happy to share with you on the phone in detail but the short answer is, some have mileage restrictions, some don't. The real restrictions are based around the vehicle being garage kept, not used as a daily driver, and not used as a errand runner. Again, I'd be happy to share details over the phone.
Just PM me and I'll call you back if you are interested in knowing more details. Or ask away here and I will check in and answer questions for you.
There are good programs out there to help protect your babies and I'd be happy to steer you in the right direction based on your particular driving habits and desires etc.