I want to provide a different perspective as I went with a 90D and I upgraded from an 85D last year. Let me preface this by saying that you will be fine with whichever option you get. I would get the P90DL if I could afford it but that's not a realistic option for me at this time. Some prospective buyers can buy a 60 because that is their price range and can now afford a Tesla that they couldn't before. Some buyers can afford whatever they want. It sounds that you can definitely afford both but wondering if the cost justifies the difference.
I would say yes to the 90D, personally, and this is why its important to me. During the winter and depending on how cold it gets, the car efficiency can go down 20% as has been mentioned or even more up to 30-40% depending on if you use heated seats, car heat, driving habits, etc.. The regen of the car is reduced also if its really cold outside. Your driving habits can also greatly affect the efficiency.
I like to have the luxury of having extra miles and to use my heated seats, put heat on max, not limit the way I drive, and not worry if I would run out of charge. If I were to commute 120 miles in the winter, I could be using 160 miles easy. With a 90% charge 60D, you would be at 189 miles. If you want to go to a friend's house, or go to the shopping mall, etc., you may think twice if you have 30 miles left. Realistically, you will probably not have many instances where this happens, but it will happen and you will think about it. Will those instances, albeit rare, make you regret not getting a 90D? Will driving more efficiently in those instances bother you? I only say that because you drove an AMG before and even though you say you don't care about performance anymore, sometimes that's hard to change
Also, if you decide on upgrading to a 75D with the unlock later on for $9,000, keep in mind that you won't get the benefits of the acceleration. So what that means is you will only be saving $8,500 (17,500-9,000) from your calculations and you will have a 75D versus a 90D without the acceleration benefits. Since you live in NC, the winter will not be as cold as it is in other states and may not affect your car's efficiency as much as it does for others.
Those are just some considerations that may be helpful.