My first thought was trailer rather than carrier too.
5 bikes is Circa ~70-80kg depending on the bikes, plus the weight of the carrier, you’ll be hanging at least and extra 110kg off theArse-end of the car…
Either a custom job or buy and old single axled trailer frame (there’s always a few knocking about for ~£100 used as M/C transports) just use the frame and fit five roof-rack carriers to it?
That would have some advantages, in that you could tweak things about a bit and would hopefully be cheaper than a fancy carrier, or a bespoke trailer… and down the line it should easily be re-salable, a custom bicycle trailer is a bit of a niche thing to be flogging….
Hmmm… Tempted to look in to something like this as a little side project myself now.