which rooftop tens are the absolute lightest
Lightest how?
From bare roof to bed ready would be the full weight, vs lightest empty tent shell for mounting/demounting...
Autohome carbon Fiber is stupid expensive but just under 100lbs (including mattress) and only requires two crossbars for mounting.
Small Columbus is under 125lbs (including mattress) and again only two crossbars needed.
Both have room to store bedding in the tent when closed so sub 30-seconds to setup for the night.
There are other lightweight tents that require a full platform rack for support, and do not have bedding storage meaning additional daytime weight is stored in car not on roof (space and cleanliness issues) but they can be lighter while driving.