Not OP, but former Tesla owner.
> Second, buying a car is, at most, one really dreadful day every three years ...
This was an interesting point for me because Tesla was my first automotive purchase. (I had an ICE car a family member gifted to me previously.) Wisdom of buying such an expensive car so early in my life aside, it means my bar for the "automotive purchasing experience" is much, much higher than the general populations, just because I was so spoiled off the bat.
> ... many don't want to go through the effort of selling privately.
I similarly gifted my ICE car to my partner, but I later sold my Tesla in a private-party sale (we moved to a major metro and were downsizing). It was surprisingly straight-forward, though that's likely because I went through a Tesla-seller specific website to list the vehicle.
> Did Tesla work with your preferred lender for financing?
My credit union gave me what basically looked like a check, and said "Whatever amount you need to pay Tesla, write here, sign there, and hand to their representative." I verified with Tesla that this was something they were familiar with and they made positive noises. Totally hitch-free when I went to pick up my vehicle.
A friend of mine did his financing thru Tesla's recommended agency--AFAIK there's no issues there, but with the ease my CU made it work I don't see why I'd use a 3rd party.
All in all, insanely straightforward experience. Like OP said, the ability to spec and see the price online, no sales pressure or negotiation, is a huge, huge win to me. I get stressed out dealing with sales tactics in Art Van over a couch--a $50k car would give me an aneurysm.