Hey! I have had the same issues with both of my children (aged 2 and 3). We set off for a trip last year down to Florida and the car ride was unbearable. I was determined to try and find a solution. So far, investing into a decent kids seat has really made the difference, as well as providing entertainment to keep them occupied in the back!
I had a bit of a difficult time deciding on what to go for, but after some advice, we actually bought two car seats by Evenflo Called the Sprocket Big Kid booster seat. Both kids are now very comfortable and the longer journeys have improved alot. You can read more about it here: https://www.mykidneedsthat.com/best-car-seats-for-kids/
Having some form of entertainment, whether its an iPad, screens in the back headrests of toys to play with can also help massively.
Hope that helps.