My daughter went through this phase but it was when she was younger. I limited trips with her to 15 min. With her dad who she was less attached to she was better so i wondered if it’s bc she wanted me to hold her instead of be in car seat, the crying would start the moment i went from holding her to car seat. I admit when nothing else worked and she was really losing it in car seat, i’ve handed her a screen briefly bc sometimes she throws up when cries too hard. Nothing really helped at the time but she’s no longer like that. Hope yours outgrows it too. Yours is a toddler so maybe wearing her out with physical activity beforehand? Maybe she doesn’t like being confined and wants to move? Sorry it’s been hard with travel.
Both my kids hated their car seat and my second hated strollers till about 1.5 yrs old. My first got over it faster than my second. Maybe try doing stuff around naps so they fall asleep in the car. I used my baby carrier a lot when I would go out and that was a savior.
My toddler was doing this over the summer with the stroller and then did better in a wagon! And then after a few months she got back to the stroller. Wonder if it's just a phase 🤷♀️ but not sure about the car seat. Maybe you need to adjust the straps or something? This happened with a family member of mine and the issue was resolved by turning car seat forward facing (i know its best to stay in reverse as long as possible but what can you do!)