When I retired I bought a D300 and, 4 years later, upgraded to a D800. I totally understand the desire to have the best gear so you “can” produce the best possible images. Get the best tools and build your skill. Would I get a D800 again? Maybe, maybe not.
As you know and from the posts here, the D800 is an amazing tool that can produce outstanding images. But it is a challenge to get tack sharp images; I mean that for me handheld it is at times difficult. With its ‘pixel density’ any motion softens the image, even with the very nice glass you have and with a high shutter speed you “need medium format technic”. If tack sharp is not so important, not a problem. Landscapes on a tripod – nothing better. But grandkids tend to be in motion. Do take another look at the D600.
My current favorite lens is the 24-120mm f\4. Not the best in low light as the shutter speed drops. Outdoors with good light, consider the new 70-200mm f\4. Lighter and easer hold for any length of time. The 70-200mm f\2.8 is better in low light but heavier. A 300mm f\4 is rumored to be on the way. You already have fast primes for low light.
I expect that you will enjoy your photographic hobby as much as the rest of us amateurs here no matter which body you choose. (I’m deliberately ignoring the pros as they are having too much fun making a living and I’m jealous. ) Cal