I have really enjoyed your retrospective look back at the D800 with all of your superb images. I look forward to more of your images with the medium format Pentax. I still use my D800 and my next big change will also be into the medium format possibly with the fuji range. Have a great rest of Christmas and thanks for sharing.
The only progression worth while from a Nikon D800 is the following (other than the Pentax, Fuji, Phase 1 and Hasselblad offerings) are
Nikon D810 - a slightly better version of the D800. I am still keeping two as I find 36mp really still good for a lot - and the 70-200 Vr2 E far too good to pass on as is the 50mm ART
The D850 or Z7ii - and Z7ii is mirrorless....
Sony A7r2/3 or 4 - but the form factor, layout is an acquired taste but in the right hands a tool of greatness.
That's your whack in full frame world. So after 8 years that is all I would shoot in full frame over a D800. That is a testament to the omnipotence of this mighty camera.
Medium format sensor wise the Pentax 645z gives nothing away to the Fuji system unless you step up to the 100mp system - I couldn't afford too. Additionally I wanted myself another SLR and some sample images from a 645z with the 28-45 sealed it for me.
I found the Nikon a wonderfully tactile thing to use, it really did become a friend and I found the relative heft over the budget APSC system I had before it really great. I thought it felt really great in the hand and the easily body mounted controls rather than have more stuff menu driven like the APSC stuff was awesome. Only the D500 can touch a D800 for feel, but the D800 obliterates it IQ wise.
It felt that the D800 felt like a real camera; a photographers camera built by photographers for photographers because that is exactly what it was and is. Hence the camera really did help my photography. Without it I really doubt I would be where I am now.
I get the same feeling from the 645z - I can tell it's going to be friend to me and I think I will buy a second one after funds recover and a couple of other lenses like the 45-85 and maybe 80-160 or the 120 prime. I'll run that lot for 7-8 years and then probably there will be nothing left to buy that is a DSLR. By then mirror-less systems will be very mature. That's maybe why the D800 has stood the test of time quite so well - DSLR's were a mature proposition even then. Look how little the came has moved on in the last 8 years with full frame DSLRs.
Some advice - a sample image from the Fuji with the 32-64 didn't seem as good in the sides - but there are lots of wonderful choices for the Fuji Mirrorless - a lovely 23 or 25mm - a 45-100 and 100-200 zoom. I'd avoid the 32-64 as the images I saw had very smeary sides beyond the realms of what is fixable in sharpening. These camera's use a 4:3 aspect ratio - I find this very pleasing over the 3:2 of 35mm format camera's so but if the sides are soft at this aspect ratio something is really quite wrong. I found I cropped my D8xx files 4:3, 7:5, 1:1, etc so won't miss the 35mm layout one bit. It was a good way of fixing soft corners which I found the 14-24, 24-70 both suffered with.