> I want it to be 100% "loyal" to me and not to the manufacturer
That is ideal, but inevitably I think there has to be a bit of a give and take there because the manufacturer is likely the most trustworthy party to handle the security of their vehicles (they have the most to lose).
For example, it's very difficult to make new keys for modern cars. Most manufacturers require that you go through them, and in fact a very secure department within the company, to get a "keycode" to make new keys. I think most people would agree that is a reasonable security benefit because it makes it almost impossible for a thief to make a new key even though it adds some extra hoops for the legit owner when they want a new key.
But Chrysler, for example, allows you to buy a new keyfob from anywhere you want, even repurpose an old one and program it yourself if you have two existing keyfobs for your car (that way a valet or service center with only one keyfob can't do it, only the owner with both keyfobs can do it).
Tesla was the first to encrypt their vehicle firmware and I believe they had to do this in order to do over the air updates, and I think that was also a necessity given the way they chose to do service operations (which also made sense since their goal was to have very few moving parts and therefore reduce the need for service center visits altogether).
I think eventually the head unit will be powerful and secure enough that they will be able to initiate many of these secure "repairs" from there. But, that will require that unit to be very locked down, likely to the same level that some of Apple's devices are (you can't self-swap the secure enclave in iPhones or macbooks since that is what manages security for those devices). The car's head unit would become the "secure enclave" for the car. Right now, the car is more of a distributed system, with no particular ECU being responsible for others... which is why you need an OEM tool to reprogram them.
I hope cars become more modular at the same time. Electric cars should last a lot longer and making it easier to swap key components, like Tesla is doing with their main AI computer, with faster better units will be great... especially if the head unit becomes the hub for the rest of the car, it would be nice to upgrade those when new advancements warrant it.