My son had reflux and I switched him to the Britax at 2.5 months because he'd literally spit up everything in his tummy and then just plain ol' acid until we took him out of the bucket. Oh yes, and spitting up acid makes the screaming just unbearable because you can smell it and know it hurts! Rear-facing Britax was better, FFing was a huge change for him. The drugs for the reflux helped too, but he never really got over disliking the car trips, and he doesn't sleep in the car either.
Anyway, when I had to drive him and he was/is screaming, I just have to ignore it. I don't turn around in my seat, hand him things or any other potentially dangerous behavior, because there is literally nothing I can do. When he was smaller, he got a pacifier in the car, and we kept it tethered to the seat on a short tether, and I'm willing to roll the windows up or down or change the music, but that's it. Experience has taught me that when he's miserable there's nothing I can do but let him out of his seat, and that's just not possible.