I own and use all 3 GFX's, and while the GFX100 offers the most advanced internals, the GFX50s is the one I go for most often when my hands are at their steadiest, otherwise, the IBIS of the GFX100 is a boon. As one poster has indicated, the 50s is easier to hold than the 50r, with its deeper grip, and while the 50r's grip can be improved with a 3rd party grip extender like the Peipro, it still doesn't equal that of the 50s. Secondly, its weight hovers between that of the GFX100 and the 50r. That said, there is apparently a new GFX on the horizon that is rumored to be a lighter, smaller and less costly version of the GFX100, so unless you are in a rush to quench your GAS, you might want to wait and see how that turns out.
Personally, I had sold all of my Sony A7r, rIII, rIV series gear because I could not bond with the small bodies, and had ended up adding the battery grip to them just to approach ergonomic acceptability. So a smaller bodied GFX100 would not suit my needs, and I do not find the weight of the current iteration objectionable, but smaller, lighter, less costly will likely work for others.