I do like that idea.Sometimes a simple box trailer is a better solution. I had a galvanised 6x4 box trailer and got hubs made to match my tow vehicle wheels, put a heavy duty axle (steel bar really) on that matched the wheel spacing of my tow vehicle and heavier duty leaf springs and mounted them under the axle instead of on top to give more clearance put a spare wheel carrier on the drawbar. It’s relatively lightweight, not too huge to store at home tucked in a corner of the yard didn’t cost that much to put together (biggest cost was 3 matching wheels and tyres) and it gave me an extra spare, or in an emergency an extra 3 spares if you ditch the trailer. A grenadier wouldn’t even know it was pulling it and with matching wheel spacing it very little extra drag even offroad in samd and wheel ruts.
Unfortunately I do a lot of desert work and I'd rather not have a trailer for the dunes. The correct answer is just to not try and fit 5 people in a car. (facepalm). There is one famous (in his own lunchtime anyway) youtuber in particular.. hi fills the entire car when travelling alone or two up. Sheer luxury!
I reckon I could easily forgo the roof rack entirely with one or two people even for remote trips. Plus I wouldn't have to go over every single thing and critically asses if it can be reduced or left at home. I would miss my awning though.
We do need someone to come up with a bigger fuel tank though!