Same here!
I had the same problem and even changed the car seat, but my son was still doing the same thing. Then I came up with a story that if a police sees him not in his car seat or without a seatbelt, the officer will take away car. That means we wouldn’t be able to go to his favorite places, like the supermarket, zoo, farm, or park, and we wouldn’t be able to get tasty treats from the supermarket. I keep repeating this story every time to ensure he doesn’t unbuckle his seatbelt, and it’s working well
Curious to see what others say, but I don't think there will be much of anything because you need to be able to undo it quickly in an emergency