OK, the crop sensor vs cropping a FF image to get the same "reach".
Both the 6D FF and the 7DII APS-C are 20 MP.
I put my 100-400L on the 7DII and get a 20 MP image at a give subject size. (Say a bird fills 50% of the image.)
I put it on my 6D and then crop to the same 50% subject size as the 7DII and I just cropped off a bunch of the pixels. Fewer pixels, for some subjects it makes no immediate visual IQ difference if I keep the prints small but get into large prints and the difference becomes plain to see.

This assumes that I take both pictures from the same distance and don't use "foot zoom".

The DOF part, it is well known that the smaller the sensor the greater the DOF at any given setting, just the way it works.

Using DX crop gives you the same result as cropping in post. No advantage as far as image quality is concerned. You are taking a camera that provides a 7,360 x 4,912 px image and turning it into one that only gives you 4,800 x 2,704 px, or 15.3 mp. If you routinely already crop your images, your image quality will not change - you'll just have fewer cropping options.

As far as you lens is concerned, you still have a 70-200, only using that smaller part of the center of the image as recorded by the sensor. The only advantage is you can shoot at 5 fps instead of 4 with the standard battery, or if you have a MB-D12 and either the alkaline battery tray or the EN-EL18 battery and BL-5 cover you can go to 5 fps.

A better option if you have the battery grip is to go to 1.2 format. You will still get a bump in FPS - to about 5 - and a bigger image - 6,144 x 4,080 px or 25mp.

It's hard to compose in the cropped view, especially if you have a lens with a small maximum aperture. All you get are guide lines, so you actually see the entire D800 frame, with the crop lines superimposed.

The 70-200 VRII does a very decent job with the 1.4 and the 1.7 TC, but you will get even better performance if you get the 200-500, and better yet if you get the 150-600 Tamron G2 or the Sigma Sport.

There is no magic shortcut to great image quality - you have a camera that can capture great resolution, now you just need to decide which of the long lenses you are going to get. Forget about a 70-300 - if you are happy with the 70-200, but not thrilled when you add a 1.4 TC, you won't be happy at all with the 70-300.

This is an image taken with a Sigma Sport at 600mm, using a D800, and cropped down to 3.9 mp (1747x2236). I made a print at 72 ppi for a finished image size of 24x31 (roughly) and it looked very crisp. Of course you will see flaws and softness if you hold it 10 inches from your face and use a loupe, but hanging on a wall, in a frame, with a nice 2" border around it, and viewed at a normal viewing distance of 4 ft or so, (1.5x the diagonal dimension), it's just fine and no one will complain that it isn't sharp.

I am sure that over the years your shooting has greatly improved compared to what it was when you first started out, I too have owned several macro lenses, between 50 and 180mm at least 8, currently I shoot with a 150mm on extension tubes that produce results like those below.... But had I started with this setup I am sure that I would have given up early... I have also shot extensively with reversed lenses and tubes... all very rewarding all presenting their own challenges...

I still recommend to anyone getting into macro that they start with something in the 100mm class because the longer lenses are a bit difficult, it is best to first learn macro and then graduate into the longer lens if you feel the need to do so.