The harness I was mentioning is called the EZ on vest. If you look it up online you can find them.

There are a few cheap solutions in the market, there is Buckle Boss and there is another Angel Guard, but if those don't work the only other alternative is a harness that clips into the vehicles teathers for the seats. They are not cheap so you would want to go through a DME and get an RX for it.

I use the graco 5 harness, is really though to unbuckle it, my 6 strong year old , isnt still able to unbuckle himself.

Recarro

When my had 6 . I used regular booster