I went to the D800 in 2011 and LOVED the IQ! 36mp and 2K video in the same package! However, two bodies and lenses got big to travel overseas and made me stick out shooting in the streets. I bought a Panasonic GX-7 and a couple of lenses and became invisible, but missed the gorgeous Nikon files, full of deal and easy to process. The 7-14mm Panasonic lens was super and with less flair than my 14-24mm, plus the size and weight were a bonus. 16mp and low light, not so good.
So I migrated to an Olympus EM-5 MkII that I won in contest (with an image shot with my D800! :-) and kept the ball rolling. So much nicer for travel and the sensor-shift for large raw files was a plus! The EM-5 was an upgrade over the GX-7 and I was happy.
But, in my experience, the Panasonic bodies were not as well constructed and service was so-so. The Olympus did the job but then I crashed off a step and smashed the first body. A replacement body also failed... But those m43 Panasonic and Olympus lenses are gorgeous and pretty well built! The IQ from my collection was never an issue.
I started to miss the IQ I had with my Nikon system, especially with low-lighting and mixed lighting.
So when the m43 bodies were starting to fail, Nikon made their play with the 14-30mm and I started the next transition.
One body led to a second and all was well. Image quality, especially in low lighting and mixed lighting was good and files were easier to craft. The FTZ adapter was a bonus and I still use several of my F-mount lenses today.
In the mean time, I started back doing some landscape work and through 24mp files do quite well up to massive print sizes beyond the math, I wanted files over the top and went with a second-hand Z7II. It creates massive files full of detail, especially with any of my three Z zoom lenses and the several adapted F-mount lenses I still use.
For day-to-day shooting, the Z6 is enough quality for 95% of what I do. For over-the-top, the Z7 files are quite nice, definitely a step above my D800 bodies.
If you were happy with your D800E and current lenses, I'd put my efforts into getting back up to speed with it. You won't need to pay the upgrade taxes as you get back into the groove, such as media cards, storage, software, newer computers, etc., that seems to come as a necessity with nearly every new hardware upgrade one does.
If you got great images before with your camera, you can do the same today for a lot less! And most of the cameras & lenses created in the last dozen or so years are still capable of creating great images with a little practice and without spending thousands on a whole new can of worms.
The best upgrade I always recommend is to invest in the photographer through training, eduction, workshops, and photo ops with the money you save by not going for the latest (and most expensive) "toys" available and using what you already have and have used.
