Hailey went bezerk until about 4 months. Then was good for a long stretch, and now recently has started fussing again (we had several projectile vomitting episodes) which really seems more behavioural.
Ellie wasn’t happy in her carseat until she was 20 lbs and I could put her forward facing. It was a long and miserable wait, so I can sympathize with you! I hated to go anywhere with her!