I have booster seats that have a 'footprint' the same size as a childs bum, they take up no more space than the child alone would. They also clip into anchors, and are very easy to remove and put in. If interested, I'll check the brand, I forget now.
In my pre-mustachian days we thought we needed a big SUV with two growing boys, and so bought a 2006 Honda Pilot. We had a Subaru Outback, but found it too small with all of our gear we regularly lug around. It never occurred to me (or was willfully ignorant?) to use a trailer and roof rack on the Subaru. I'd sell the Pilot now and downgrade, but makes no sense to do so. You have to pay a 13% tax on all vehicles when you buy, better to just keep it. Fortunately, it is only used a few times a week for family outings, lessons, etc. As well as road trips and camping. Never rolls without at least four of us in it, and often more, we hike with friends/family and all pile in (it's an 8 seater).