My main problem with the DX for macro has always been light. I've constantly been hunting for a stop I could take here to invest there. ISO was the main reason. The very maximum I could use was ISO 800 and that ruined plenty a shot. When shooting handheld 1/320s to 1/400th would give the best results since I'm shooting that close, any imperfection just stands out. I could gain some when using a tripod but the reality is that unless you're shooting flowers, tripods aren't very practical for bugs. At least not for fast movers. A flash might help too but also isn't always practical and you need quite a system to shoot flying bugs that require 1/1000th or more.
When I went into the woods here, macro was close to impossible. The overgrowth eats most light and combined with a lens pointing downward, a mushroom in some clearing at low shutter I could manage but anything moving fast was "look at" only.
The FX I can take into the woods and shoot. I'm sure I would have a hard time pulling of a fully detailed 1:1 macro at ISO 6400, especially if I need to crop that but when being able to use the full shots, I can get away with quite some ISO in comparison to the DX. Evidently, the D750 is pretty decent at it.
I don't know how your D700 will perform but it might turn out to have advantages for certain shots.
For all other things DX vs FX matters little. Higher dynamic range and such is great but only matters when comparing. It's the same with DoF, FoV or Bokeh. Those differences are only obvious when putting two identical shots next to the other. For a shot on its own, it would matter little. None can look at a shot and say "that's typical FX or DX" because of those. At least not when having zero EXIF information.
It's only now I have the D750 shots next to my D3300 that the differences are obvious and I can point out what I like more in the one or the other. Before that, I was perfectly fine with anything the D3300 shot. And most of those differences have to do with the dynamic range they grab. But it's 500$ vs almost 5 times that price. I'd be sad if there wouldn't be any difference.
Here's an example of two rather similar shots. Keep in mind both use a different lens too so sharpness should be ignored to a degree.
This is the bridge shot at night with the D3300 and a shot I was perfectly happy with.
This is the same bridge, other side of the canal shot with the D750. Yes the clouds make a difference but the range is what makes most.
I also lost quite some detail to clipping for the D3300 (bottom of the shot, path) while this wasn't the case for the D750. For both, I was standing in a pitch black area. The D750 was shot with LE noise reduction off. Not a good idea but it still did well.