@Niomi thank you!
If it’s an option to put your dog in the front (that’s what I do with mine) then just make sure front passenger airbag is disabled! Good luck for your travel x
We’ve just done wales with our LO just turned 7 months yesterday it’s 5.5 hrs but we had 3 stops each way. She slept most of it to be fair and I sat in the back to entertain her and dog was in the boot.
They shouldn’t be in car seats for longer than 2 hours a time at this age. I always sit in the back with my son so a bit different but also 7 months old - I just take his sensory toys to keep him entertained, but he usually falls asleep in the car anyway and likes looking out the window at everything! Things like rainmaker, rattle toys, etc, nothing that’s too noisy for my partner driving. We frequently stop at services every 45-60 mins anyway for nappy change, legs stretch and get him out for a quick play or feed… I would suggest just breaking up your journey and stopping half way through to get her out and if she’s a calm baby she may just settle down with the dog and nap anyway :)