GFX100 is a very good system upgrade from FF system. I was waiting such kind of camera for a long time (they did exactly what I wanted - many different things) and I also was considering a7rIV but decided to go with gfx100. After Sony announced a7rIV and seeing sensor tests results I decided to keep a9 and a7rIII, as well as my old 1DXmII and added GFX100 to my gear set.
So far no disappointments - focusing very quick, manual focus is much so more convinient and more precise then anything I tried before, face and eye AF works fine, image quality is excellent, per pixel SNR and DR is better that for the same pixel size of a7r4.
IBIS is very good, video quality is also very good, weight is less than 1DXm2.
Very easy and convinient to use.
Unless there are some new disruptive upgrades in FF sensors technology soon, in coming years GFX100 will be superior to any FF cameras.
My old 6 years laptop handles GFX100 RAW files quickly and easily when using C1 ver.12 , some problems with PS6, as when sending image from C1 to PS layer size is huge - around 300-400mb and PS PSD max file size is only 2Gb, that limits number of layers which is possible to have for intermediate edit work.