Honestly, I wouldn't overthink it, and would just do it, sit in the back and make 1 or 2 stops.
The evidence behind the 30m is very shaky, and mostly relates to poorly or premature infants. Before this 'rule' came in, no one would have thought about it twice, and I don't believe the number of babies dying in cars has changed very much. Being in a car for so long IS riskier, but virtually all of that risk is because being in a car is inherently riskier than not. You are more at rush on a long journey than a short as well.
We did a couple of journeys of about an hour within that first week, went through town with lots of speed bumps etc, and it was fine at my section. No pain whatsoever..
Note, the lullaby trust does not give any time limits in it's car seat guidance (https://www.lullabytrust.org.uk/safer-sleep-advice/product-information/car-seats-and-sids/)
I think it's one of those 'rules' that you have to test with common sense and make your own mind on. Like the 'stay in the room with them when they nap', doesn't mean lugging a moses basket around so you can pee. Or making each bottle fresh from water that has cooled for 30 minutes whilst simultaneously feeding on demand 😂
Often the guidance takes no account of practically or maternal wellbeing. But then you get postnatal anxiety creeping in, and mums peeing themselves because they don't want to leave their baby for a minute to go to the toilet, or getting stressed that their baby is going to die because they're in a traffic jam and baby has been in a car seat for 45 minutes.
It's useful to know why these rules/guidelines have been given, but feel free to find your own way around it to decide how strictly you will stick to it, and if there's any mitigation you can do when you go your own way.