I think the best solution here would be to outright disallow (by law) companies from manufacturing devices in a way in which they retain more control over them

I think the best solution here would be to outright disallow (by law) companies from manufacturing devices in a way in which they retain more control over them even after the sale to the end user has been completed. They can have either the same level of control, less, but never more. As it stands right now, Apple and pretty much all Android manufacturers have more control (Yes, I said Android too. Good luck trying to mod the bootloader itself or gaining access to the TEE) This would be a great thing because that way you don't force any company to modify their app plattform or change their normal user-facing software in any way, yet those who wish so can do with the device whatever they want in the same fashion the manufacturer could before (and even after) the sale.