I find that emptying back pockets makes a big difference after an hour or so. As I've gotten older, I find that 1 to 1-1/2 hours is my limit without a break to walk around, more so on constant speed road than where speed and conditions are changing.
Don't know the rules here, but know that the EU does not let commercial drivers go more than two hours without a substantial break, and that's not all about staying awake, it is partly about occupational health.
Next step would be working on the seat configuration, which is something better sorted out before we even buy a vehicle. Road tests need to be at least two hours, 10-15 minutes won't tell you it the seat and controls fit your body well. I've sold several vehicles after 2-3 years because I couldn't sit in them on long trips, kept others for 12-20 years because they did fit me well.