I do remember going through a stage when the restrictiveness of the car seat was an issue and I have noticed that my father and I are uncomfortable with seatbelts on.
I always thought of stages as about three months long and then (in layperson's terms) maybe there is a change (because the brain grew and changed?). So, I would hold off on the car seat and stay home and in the neighborhood and wait until the child's needs met the activity and the activity met the child's needs.
For a very high sensitivity child, that might mean the family making arrangements somehow for the child to be around home until the car seat is no longer an issue.
I would try a different car seat. I would use every kind of distraction - music, toy, book, portable lap desk with paper and crayon. (This is one of the reason's people don't realize how exhausted the caregivers can become. We can't just grab our child and go.)
