Grim, allow me to answer some of your points.
The problem with allowing shit-mobiles masquerading as real cars on the roads is two-fold.
One, if the car is capable of highway speeds, you will not be able to ban them from the highway. It simply won't work. They look enough like real cars to pass, people will use them on the highway regardless of legality, and it's going to be a real mess because people are idiots. The only way to get around that is to limit their top speed to ~35, which is already a thing that exists and you can go out and buy such a vehicle right now.
Two, the choice - now, you may say, well, we can simply sell these less safe cars and make sure everyone is informed that they're rolling death traps, and of course nobody would spend their money on them when they're for highway use. That's all very well and libertarian, but it is simply not how life works - dealerships will be convincing people to buy new instead of used, lying about the danger involved, and so on. People will convince themselves that they "deserve" a new car and would rather have new instead of used for the same price. And so on and so on. This is why regulations exist for many safety-related items like seat belts - people are fucking idiots and eventually we all get together and decide that enough is enough, people are dying for no good reason, and it benefits society to make it stop.
So we have minimum acceptable safety standards for four-wheeled production cars.
Mind you, nothing stops you from buying a kit car or similar - something that basically has a tube chassis (or sometimes fancier), suspension, drivetrain, and nothing else. Super light, super fast, super fun, and it'll kill you. That's fine if you build it in your garage or if you buy it from a boutique, limited production manufacturer. These types of cars are allowed specifically because anyone buying one or building one has a bunch of money and is knowledgeable about cars and has done research. Normal families don't buy them, kids don't turn into a bloody mess inside of them, because they're toys. There are no dealerships trying to convince families to buy them, nobody is pretending they're anything but a fun death trap. Much like motorcycles, it's very much a "you do you" situation that works only because the numbers are so small, because the drivers are all enthusiasts, and because they're generally used only in the best situations (dry ground, warm tires, good visibility, low traffic.)
As for winter tires, winter tires are not a safety item in Florida, so nobody is mandating that you use winter tires in Florida. It's that simple. Winter tires are a safety item in other parts of the country, and guess what? Their use is mandated! Here in the sierras, you have to have AWD/4WD, proper all-season tires, winter tires, or chains, depending on the snow severity. In other places, winter-capable tires are required by law between certain months. Bringing up Florida is a red herring, a strawman - Florida doesn't see a significant amount of snow, ice, or even sub-zero temperatures, so it's not an issue to drive on basic all-seasons year round; with a careful foot and careful distance keeping, even high performance summer tires do just fine in the coldest Florida winter despite having the traction of rubber pucks. Try that shit in the sierras with snow on the ground and you'll die. Hence laws.
Horse-and-buggies are a weird case - they're allowed on the roads essentially due to the first amendment rights of the amish. Despite that, deaths (especially in years past, when they weren't decked out in lights) were common. They're destructive, dangerous, and shitty (literally), and the only reason we really allow them is because of weird religious bullshit and lots of tradition (and shitloads of road signs, and fairly limited impact because very few areas in the countries have buggies on the roads.)
So I guess if you want, start a main-stream religion that demands that you drive shitty deathtrap cars on the highway.