Technics SL-1200GR setup question

OP, I think you are fine.

I now have two arms with VTA on the fly, adjust while listening, and, like prior TT arms, I find a slight tilt down is best.

Prior Audio Technica TT was the same way, arm at zero/lowest had a slight tilt down. Sounded terrific.

VTA is really to get SRA correct, Stylus Rake Angle, tip in the groove: +/- 92 degrees

https://www.ortofon.com/hifi/products/hifi-cartridges/mc-a95/set-up/#:~:text=STYLUS%20RAKE%20ANGLE%2...

You can move your TT anywhere, a position where you can view sideways thru a magnifying glass, set arm height, then put TT where it belongs. 

I bought one of these, easy to check VTA which is easier to see, then view Stylus tip with magnifying glass.

https://www.amazon.com/gp/product/B01HF9PEA4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I have the 1200 G, and I believe arm adjustments to be the same as on  the GR.
The issue audionoobie raises is one I have myself sent requests to Technics' support about, but never received any answer. The fact that, although there is on-the-fly VTA adjustment on the table – it does not let one lower the arm down to the parallel level or below. I do not understand that anyone can insist that parallel is always right (or even the 7-9 mm raised back end). Correct level/angle varies from cartridge to cartridge; dependent on stylus rake angle, cantilever angle and compliance, cartridge height. I switch between 2 cartridges (Soundsmith Zephyr MIMC * and Benz Micro LP) and both offer better bass (not just more, but clearer and better defined) when I have set the back-end of the arm lower (i.e. below parallel). My prior deck (VPI Super Scout) had a much larger adjustment range than the Technics arm. Raising the back does not only cause the bass and lower mid-tone to become less distinct, but also carries a high-end distortion penalty – like sibilance. I have seen others complaining about this on the Technics, so it's obviously a design error. I have tried to work around the problem by using two mats at a time (cork/rubber and/or cork/leather) but it does not give me enough as the spindle is very short, so this leaves no room for a clamp or a record weight. I wish Technics would own up to this problem and come up with a solution.

I have commonly found that lowering the pivot with respect to the headshell does shift tonal balance toward the bass and that raising the pivot above level with respect to the headshell does shift tonal balance toward the treble, which, as you say, can enhance sibilance.  There are good reasons for this. Using a thicker mat doesn't help enough with VTA adjustability on the 1200GR?  Are all the G series models similarly affected? At least Audionoobie knows he has company.

Technics EPA-100 is much better tonearm and can be adjusted properly on any turntable.

Regarding G or GR stock tonearm all you need is a thicker mat or spacer between cartridge and shell and then tonearm will be parallel if you wish to.

make sure u are using the stock headshell, it puts the cartridge lower than other aftermarket headshells. 
to level the assembly level the platter in all directions, put a small round level on top of the flat part of the headshell (temporarily adjust vtf to compensate for the weight).  then adjust tonearm height dial to level the headshell. 
use of a thicker mat helps with my cartridge, i use a 4mm herbies, stock mat is 3mm. 
also i believe many use a spacer on top of the cartridge to lower it further and add mass, which helps with the system resonance.  
it looks like a denon cart is about 2mm too short to be level on a stock GR at the lowest tonearm height