2020 Prius roof rack spacing options for large canoes?

Hi all!

I'm looking for a boat like an Adirondack Wherry: a 20' long 90-lb rowboat. It's basically a lot like a big canoe: not a monocoque like a kayak, so probably less stiff, more windage, and longer than most. I also have a Seaward Passat that would be fun to carry---it's a very nice 22' tandem kayak. It's a lot more load than a single kayak!

The Thule roofrack for my car fits beautifully and seems sturdy, and is ostensibly rated for my load, but it is positioned with pins on the feet that fit into receiver holes in the doorframes. That means that despite the nice long roofline, Toyota saw fit to tell us that the only possible rack spacing is a mere 29". Thanks, Toyota

I'll need front+rear tie-downs (except possibly with option 4, below). I'm somewhat at a loss as to where to attach them: the usual suspects under the hood seem to be flimsy plastic or thin sheet metal, poorly suited to attaching anything. Suggestions?

For sea kayaks and long canoes etc, what has been your experience? How necessary have you found it to increase the rack spacing---either to support the boat vertically along more of its length or to better control side-skewing forces? How have you done it?

I'm thinking of some possible options:

1. Use as-is, being careful to make the fore+aft tie-downs snug enough to keep the boat from bouncing, but loose enough to present no risk of buckling the boat. I don't know how much of a risk that is.
2. The roofline is fairly long, and I could bring the rack spacing up to around 48" by either cutting the positioning pins off the rack feet (are they really important?), or drilling new receiver holes in my doorframes. However, the undercut of the doorframes changes a little where Toyota decreed that the racks will go: how much should I care? Is this decisive?
3. If my main worry is buckling the boat's hull due to all the upward force in the middle and all the downward force at the bow+stern, then I could try some kind of custom frame (wood, 80/20, etc) to support the gunwales further from the centre. A useful plan?
4. There are also trailer-hitch rack extenders such as the Thule Goalpost, Yakima Longarm, etc. I'd need a hitch---ideally a 2" hitch---and a big piece of extra hardware, but I'm guessing it would be very secure, and even let me skip the front+rear tie-downs. I'd probably need to play some games to allow me to open the trunk with such a thing installed, but could be done. Any luck with these? Any favourite hitches and rack extenders for our cars? Or is this overkill?
5. Has anyone had any luck making front+rear tie-downs incompressible, like perhaps making them out of ABS pipe? That should reduce the buckling risk, but I'd be curious where and how to attach them. Over-the-hatch bike carriers seem popular and might be a good starting point...?

Thanks for any thoughts!
-Ben