We did the stopping every 2 hours so she could get out of the car & roll around on a blanket thing. We found parks & would go on the swing etc. Have lunch and just took our time. Because at that time I was still bf, we would stop for a feed etc, and even gave her a comp bottle in the car which she wasn't okay with LOL.. She was okay. After around 5-6 hours in the car she got cranky and we had to stop for a full 45 minutes just to let her have space outside of the car, and that helped us heaps too. Our usual stops lasted 20-30minutes but having a bit longer when she was awake was great.

We did our driving routine like her sleep, so we left the house around 30 minutes before her sleep was due (ish) and then when it was time for her to have a feed we stopped, then played for 10-15 minutes and back in the car until she woke up again. She slept less so for only 45 minutes or so. But she slept 3-4 times during the trip too...

HTH!