@Waterbird (and in general)
Correction on the lift devices. I had this wrong myself in an earlier post, sorry if I started an urban myth. Rookie corrected me and she is right. The Hullivator mechanisms, the most common lifting aid from what I have seen, and add about 18 pounds per unit thus a total of 36 pounds. Otherwise they rest on the usual cross bars. The Yakima Show Down rates at 21 pounds, and similarly allows waist high loading, but there are no gas shocks and no assistance in getting the boat from the side to the roof Hullivator takes over 45 pounds of weight as you lift, more than the weight of the units.
On the side where I have the mounts for a second kayak, after a fair amount of messing around I settled on saddle plus glide pads as the easiest to load, I use the Amagansett Roller Loader. It clears the rear spoiler and, while I have not made a home-done version, I don’t see why someone could not manufacture an equivalent if the roller loader folks could do it. Just copy the same proportions of wheel and stabilizing foot. Some units I have seen were manufactured with replacement parts for kids’ bicycles. Added advantage is you can get parts that are pink so they are easier to see in a dark basement.
The Roller Loader is not in the car unless I am kayaking (or too lazy to fully unload). So I am not typically carrying around any extra weight to speak of.
The biggest impact on fuel efficiency in any case (and at times noise) is the arms, saddles etc that you add to the cross bars. This amount of weight of these units is not going to chew up more gas. So stackers or anything that can fold flat are going to get you more advantage.
Also, the Hullivator units are the only version of a load assist device that accomplishes what they do which are designed to easily remove when not needed. (Hence losing the weight.) I don’t swap them out as often as some, like who do it before and after every trip. But this season I have taken to removing them more often when I am not carrying a kayak. After some initial colorful language I have found it is fairly easy to remount them as long as I have a stool of the correct height.t in use.
Both the Thule Hullivators and Yakima Show Down still involve the initial lift into the arms, which can be nudged in one end at a time but frankly goes best if you commit to a dead lift from the ground. As a boat gets bigger, this is still an imperfect thing. As I indicated higher up, I am probably carrying a bit less weight at maximum using the Roller Loader than the lift into the arms. But the boat can’t slide off angle onto the ground again either, and the initial strapping is much easier. Plus when I realize I have forgotten to remove something from the boat (last night’s paddle it was my spares), I can easily just drop it down again to waist high and retrieve what I ditzed out on.
Honestly, there is no system out there that is not going to require a bit of strengthening and practice by a 5’3" tall 130 or less pound female. Short of a folding boat. But you need a certain amount of strength just to paddle and move the boat in and out of the water. Having had to recover that strength a couple of times or so, I can say that the strength needed to load with either of my options is about what is needed to get the boat carted to and from the water. Or perform one of the more physically taxing self-rescues with reliability.