If it was an grounding issue wouldn't all resolutions be affected? I don't think the monitor is grounded to anything special. There is a spring-like wire going around the monitor that attaches to the neckboard.
It could still be grounding, just the effects not apparent in all modes. However, that is just a theory. Leave that spring-like ground wire from tube to neckboard, you need that there. Usually there should be nothing connected between monitor chassis and ISO except 2 power lines (active and neutral).
One way to test the AC ground interference theory is to plug in something to the same powerboard/outlet that has a powerful electric motor, like a mixer/blender or a drill. See if running that makes a lot of interference.
Keeping video signal wires away from high voltage areas of the chassis like the flyback is always a good idea.
I have a suspicion that this moire effect is common in Nanaos. Many Many years ago I had an Astro Cab (like a Candy) with a 29" Nanao monitor and, iirc, I am sure that I had this same moire issue. I went over everything, including grounding, and didn't get any better than small improvements. I was using an ArcadeVGA and JPAC.
My Nanao was one of those dual-res ones that could do EGA (25khz) as well as CGA, but it was always set to CGA anyway. Can't remember the model, but it would have been ms8 or ms9.
All this leans toward Andy's diagnosis earlier, that the moire has something to do with a mis-match between phosphor dot-pitch and game resolution. Maybe the standard Nanao tube has a weird phosphor mix/layout/pitch to better suit EGA? However, I don't think that there is anything wrong with your convergence. Pic look good in that regard. Your monitor is not "too sharp". Leave the convergence rings alone.
Fiddling with focus/convergence might minimise moire, but only because it is harder to see everything through the fuzz.
If dot-pitch/resolution mismatch is the cause, you might find that the problem disappears with another tube! Anyway, you can't really test that without a matching tube lying around...
I also suspect that it is not CRT_emudriver causing the moire, as ArcadeVGA causes it too (if I am recalling correctly above, please test if you have an ArcadeVGA lying around).
Which brings us back to the JPAC. Maybe you could try running signals directly to your Nanao, bypassing the JPAC, to see what happens? The picture will be faint, for sure, but am interested to see what happens to the moire.
Evewalker notes that the problem goes away when running a PCB (ie no JPAC) vs MAME:
Here are some logs from my setup, both using Standard Resolutions, and Super Resolutions installed.
Ketsui has no moire effect at all. Donpachi has a severe moire effect when scrolling using both Standard and Super Resolutions installed. Using the actual PCB gives a functionally identical picture except for the lack of Moire effect and wavyness.