The CCD backs have a different look from CMOS cameras/backs. They're beautiful at base ISO (which is often around 50), but fall apart by ISO 400 (and aren't great at 200, either). I've looked at them occasionally, but size and weight has always ruled those systems out for me. The IQ180 in particular has stunning base ISO detail and dynamic range, even on very large prints.
I've considered the Fuji CMOS cameras much more seriously, which use the same sensor as the IQ x50 (and bought a Z7, which has the same image sensor as your D850, instead). I looked very closely at large prints from the Z7 and 50 MP medium format CMOS , and found that I mostly couldn't tell, staring at a big print from 6" away. If there was any advantage, it was in favor of the Fuji, but it was much smaller than, say the difference between a D800e (probably a D810, too - but I don't have a lot of D810 experience) and a D850/Z7. I was coming from Fuji APS-C most recently, and the difference between 24 MP and 46 MP (plus a sensor size) was night and day, while the difference between 46 and 50 plus another size was tiny. Remember that, to gain the 4 MP and the extra sensor size, you give up one significant sensor generation - it's the same technology as a D810, not a D850.
This does NOT apply to the GFX 100, which has a current-generation sensor, plus it has a radically higher resolution than anything below medium format. According to specs and early reviews, the detail and dynamic range are amazing (body may still be a little quirky, although Fuji's good at fixing such things in firmware)
I haven't had my hands on one yet - would love to try it, although it's too bulky for most of what I do (wish it was in the 50R body). Nobody's had their hands on a new Sony A7r IV yet, which uses a 24x36mm version of that same sensor at 61 MP. If the real performance is in line with the specs, it should perform slightly better than the older 50 MP medium format sensor (the 40-50 MP Sony 24x36mm sensors are extremely close, and this is a slight improvement - should be enough to kick it over).
Everything I talk about here except for the IQ180 and the GFX100 is in a very narrow (and superb) range of image quality. What'll it take to tell the difference between a D850, a GFX 50 or IQ150 and (presumably) an A7r IV? Put the IQ160 in there, too - except that it can't keep pace as ISO rises. A really close look at a very large print, or pixel-peeping at high magnifications... In most cases, "upgrading" from one to another will be invisible.
Upgrading from 24 MP to any of these will be very visible if you print big - I did it 8 months ago, and am amazed at the difference in my big prints. Upgrading from one of these to the GFX 100 will probably be very visible if you print big enough (what's "big enough"? - without experience with the GFX 100, I don't know). Do any of our lucky GFX 100 owners have a similar print from GFX 100 and anything in the next group down (GFX 50, X1D, D850, Z7, A7rII,III or IV, S1R)?
You have the additional consideration of flash sync... If you don't mind the AF, what about an X1D? There should be originals for sale at decent prices, as some photographers upgrade to the X1D II. It's a sidegrade from your D850 to the older 50 MP sensor, but it does have leaf shutters. Expensive lenses, but it just might be what you want?
The body and back systems are huge - the spec sheets don't do justice to just how big they are. They look like they're only a little bigger than something like a D5 or a 1Dx when you read the specs . When you handle them, they feel more like twice that - and the mirror slap is a real recoil. The X1D is a really nice camera to handle, although the interface is odd. The GFX 50S and R are also nice cameras to handle (the 50S feels quite a bit like a good-sized DSLR, while the 50R feels like a big rangefinder - unusual, but not bad at all, especially with the smaller lenses).
