We had a lab that hated to ride in the car. I think it stemmed from a situation as a puppy where my wife had to slam on the brakes, and the puppy went flying into the center console. In any case, she would not relax in the car, ever. Would pant the whole time. We tired the thunder shirts, gummies, Benadryl, everything. She would get in the car, but didn't like it. Would typically not potty at rest stops, and would not drink hardly at all on trips. And I'm talking long road trips- 14-18 hours.
All that said, we just kept on trucking. We did our 15+ hour drives 4-6 times per year, plus who knows how many 2-5 hour trips, and always brought her. We worried the first few times, then just learned it isn't that big of a deal. Sure, she didn't like it- but there were no negatives otjer than her not relaxing. She would eventually get tired and lay down to sleep, but only after a few hours. Dogs are WAY tougher than most people realize, and do not need constant comfort like we think they do. Some dogs just always get nervous and don't like cars. Sometimes there is t anything you can do, other than stop worrying about it. They will pick up on your stress and it makes them worse.
I would also work on crate training her. Many people also give up on crates, when it's actually the separation that stress the dog. Get a crate in the living room, and start training the dog to see it as a positive when you are in there just watching tv and hanging out. Normal day, normal environment, nothing new or stressful. Door open, don't even close it. Lots of treats. Just work with the dog until they will go lay in the crate voluntarily. Then work on door closed. Then work on door closed and you leave the room for 30 seconds, then a minute, and build up. Eventually, you can work on the crate in different places. Once the crate is a safe space in their mind, it will dramatically help with anxiety when you bring their safe space along into stressful environments. It takes dedicated work though, but is worth it.
Another thought- you may be stopping too often. If the dog is stressed, every time you stop, get out, and get back in you start the cycle all over again. You may not be giving it enough time to settle down and learn it is all OK. Dogs can hold their bladder at home longer than an hour, they can do it on the road too.
We were determined that our current dog would be a better rider, and she is. We did loads of work making the car exciting for her, taking lots of small trips, taking her with us to go get gas or run quick errands, etc. She is sad when she DOESN'T get to go in the car, and is a great rider.
