Most important us to make sure you’re next to your baby so you can see them at all times, and get them out of the seat and moving very regularly because they can suddenly start to struggle to breathe in the slumped position. Rare but can be very serious.
Recommendation is to stop for 15 mins every hour, or if asleep, wake them after 90 mins at the latest for a 20-30 min break.
Although some are even more cautious: www.babysafeltd.com/safety/2-hour-rule/
Good luck. We’ve done a journey like this when ours was a similar age, but took pretty much a whole day over it