First step is probably just to jailbreak them. And then fight car industry lobbyists that try to ban said.
Discussion
Yeah, most can probably be fixed on a firmware level...
You can get a PhD in gulag tech cars, or you can get a pre-gulag tech (i.e. old) car.
Which won't be allowed into any EU city within a decade or so. And between maintaining a Battlestar Galactica style old timer and reverse engineering a future modern car, I'll probably enjoy the latter more.
Though maybe there'll be some pre-gulag electric cars where you can retrofit whatever solid state fancy batteries they'll have in 2050.
I agree that this is one area, where going low tech is a good choice. You aren't loosing any important functionality.
The only non-nonsensical counterpoint I’ve come across is later models may be rated as safer than older models.
But they're going to argue that custom firmware is bad "for safety". And they're not necessarily wrong, especially if they deliberately make an entangled mess where safety-critical control systems share code paths with their spyware.
There are opensource car computers and software