We carry a 17' canoe on the roof of the truck. 1 Yakima bar at the rear and a front "T" in the front hitch of the truck. Had the T made higher for better visibility. If I had it to do again I'd just have the "T" made at a local welding shop. The longer addition I had fabricated it of MUCH better quality.
We also camp with several families who carry multiple kayaks. Think up to 6! They get a two person plus three others on the roof racks with no front "T". Yakima racks all around. Have not heard a single complaint. The other smaller boats (kids) go inside the trailer.
You didn't mention bikes, but we (as well as others we camp with) have a 2" receiver mounted to the pin box on the 5ver. That carries a 4 rack bike rack. Just clears the trucks. And leave the bed free for firewood, generators, toolbox or whatever.
Yes, we carry a lot of stuff. Your yaks should be just fine on the truck roof, especially if you don't carry 4 bikes off the pin box.