Interesting...
So, what's wrong with the D800 and how will this be better?...Did you not 'bond' with the D800?......Are it's files too large?......Too heavy?...??
- Weight obviously. The difference between the D800 and the D780 will probably be around 250g.
- Size, and this is a problem I had when I had the D810 too. Too big for my hand and too big for my most used travel camera bags.
- Video. Earlier, I thought I would only use mirrorless for video. That has changed, and I will use DSLR cameras for video too, but the D800 is not a good video camera.
- No flip screen. I use that a lot, for low angle shots, when I need to be descreet etc.
The D800 is great for what I bought it for, mostly industrial stills, but I think I will be happier with a more versatile camera. The D850 is also an alternative. It has a shallower body and a more distinct grip than the D800/810, although it's still large and heavy.
I also did the mistake on going for zoom lenses. I realise now that I prefer primes for DSLR cameras. If I can afford it, I'm aiming for a 20/35/58/105mm combo.
It all depends a bit on the viewfinder of the D780, particularly the diopter adjustment. I don't like to shoot with glasses and I use +2.5. That's more than most recent Nikon viewfinders can be adjusted to, but with the circular eyepiece of the pro cameras, a +2 or +3 diopter lens can be attached instead of the standard lens, while on the rectangular viewfinders, that's an add-on that limits viewfinder functionality. Interestingly, older Nikon bodies, even my F80 and my D300, and all my Panasonic cameras, can be adjusted to at least +3.
Why DSLR then?
After shooting mirrorless for nearly 10 years, mostly in combination with DSLR bodies, I have noticed that my eyes get "tired" faster when shooting using an EVF. It's a similar feeling to what I get if I work too long with the computer in the evening, although not exactly. Then there's this feeling of added "distance" to the subject. It's there, particularly when the camera is moved or zoomed fast. Then there's battery life (and most battery saving features on mirrorless cameras affect shooting in some way or another).
Oh... and I actually like the sound of DSLR cameras.
I will still use both technologies though, and to keep the door open for two technologies using the same lenses and batteries, Nikon and Canon are the only games in town. Both even give me the option of using film too with the same lenses, although not with Z-mount lenses.
There's been a trend lately to dump DSLR completely when going with mirrorless. "Last year I loved DSLR cameras, but that's soooo 2019, so this year I hate them!". The world is becoming increasingly black and white.