I always loop my straps over the cross bars BEFORE I load the boats. I never have to “work over the car.” Which is a good thing since I’m only 5’ 4" with short arms and seem to grow shorter each decade. Since I’ll be 71 in a couple of weeks I’ve been officially a “little old lady” for some time now and solo load 90% of the time.
I use 15’ straps and lay them out towards the hood and and trunk/tailgate of the car. Once the boats are on the rack I just have to walk to the front and rear of the vehicle and toss both ends of the straps over each boat, pull the free ends until the buckles are in a good position and fasten them through the near side of the Thule rack and, in my case, through the Mazda’s lateral factory bars to which the Thule is clamped.
Once all the buckles are snug, if I am feeling lazy, the excess strap lengths just get slammed in the closest car doors. For a long highway trip I will instead wrap the ends multiple times around the rack and secure the ends in a tight knot.
Trying to clamber over and reach the bars behind the kayaks once they are on the rack is a giant pain and one I can always avoid with this method.