If we do something like that, we can also reduce the size and change the material. Just for ease of use, maybe something thin and metal, like this?

Maybe just use an rfid coil in the ignition and turn a key to start like immobilizers have done for like 20-30 years at this point.

Better yet use physical contacts in the key and use an on key cryptographic controller to do public key authentication.

That way key material never leaves the key itself after being manufactured making it basically impossible to steal the car without the key.

Oh god I always hated those cars that start by pressing a button. Fuck off let me turn my key. I like the thing where the car unlocks automatically if you are next to it and all that but for the love of god the press button to start car thing needs to die.

Yes, but you can usually just disable the proximity unlock feature on the car itself if you are worried about it

They pretty much already do this. Most modern fobs turn themselves off if they've not moved recently, which protects against relay style attacks.

they don't need the key fob any more.