Our experience with wheels in back when we tried saddles and rollers for a while was that the rollers needed more frequent checks when traveling to stay tight. They do concentrate any risk of minor indentation more than a larger surface by concentrating the point of impact. Thule had something they called glide pads in back, though that system is apparently no longer available, that had the same idea for helping load but weren’t rollers. Link to the old stuff here, you may find someone else doing the same thing or a newer version. https://www.rei.com/product/737629/thule-glide-and-set-kayak-carrier
These help if you are sliding the boat up from the back, wouldn’t make much of a diff if you planned to use a bar extender from the side. I am probably not the only person who found a set in my basement though, if you like them I suspect you can find them used.
Personally, for a plastic boat I would skip the saddle thing entirely and go to stackers. We went back to stackers even for the composite boats after trying saddles for a while, they were more secure and altogether less fuss. And with a plastic boat you wouldn’t have to pad the bars like we did for the glass boats.