Not sure exactly which of the above images is your Thin goal (the Fat one is obvious), but in any case it seems the changes in the rig are simply to increase/decrease the distance between shoulder and hip/thigh joints. This will work with a single control by using a Transformation constraint on the Shoulder & Thigh bones. The same control could also be used as a driver to shift from one Shape Key to the next. As the control is activated incrementally (i.e., with an animation curve), the bones shift position and the “thinner” shape key is gradually enabled fully. The main animation curves will not need to be fussed with.
Looking at your file, creating a shape key that differentially alters the different body parts is straightforward. The torso can slim down differently than the arms & legs, using a simple Scale transform. Shifting the bones using the control will also draw the extremities into place pretty much automatically so that the arms & legs will need only a little editing for the Thinner shape, again, using Shrink/Fatten, which can be applied to only selected portions of the mesh.
Is it an easy, simple and no-work solution? Nope. But those do not exist anyway.
BTW, why is everything triangulated? That makes editing a lot more time consuming. Maybe better to triangulate after all the mesh editing is done.
Minor correction: The Leg Control.Location will need to be edited during the slimming process due to the IK constraints but that’s a very simple shift on one axis that can be done in the Graph Editor. It could also be driven but it’s simpler and gives more control just to edit the curve manually.
PS. Much of the above can be avoided by simply keyframing the bone shifts & Shape Key shift manually, a matter of setting a few keys. Automation is always more complicated to set up.