I also have spokes in my flywheels on my 6/1. I couldn't use my shop heat/light lamps with it as the flicker bothered my epilepsy pretty badly, so I did some work on trying to fix it. I could not find heavy flywheels so designed my own AVR specifically for flicker reduction. It works adequately for my 250watt each heat lamps, below 100 watt bulbs still have bothersome flicker.
I agree that heavier flywheels and more mass on your generator will help greatly; someone measured their SOM flywheel unit with a low pass filter circuit I made to look at flicker objectively, and just the SOM flywheels worked as well as my custom AVR running on my ST3.
An AVR on the harmonic winding will do very little for flicker; there isn't enough excess power on the harmonic during the compression stroke to compensate for the voltage drop.
For flicker compensation electronically, the ST-3 would be preferred, as it has less inductive lag in responding to field coil current increases. You must then use the mains output for excitation instead of the harmonic, so that excess power is available during the compression stroke.
I have a simple version of my AVR that I published publically that can be used, which has a PCB (layout available) or could be wired by hand. It costs about $80 to have 3 boards made (minimum order) due to the setup charges. Send me a message if you want it.
I can't recommend CGG's AVR for flicker reduction sans harmonic; I tried it on my new neighbor's setup and it was pathetic; the unbalanced load made the ST-3 moan and vibrate, and it could not start motor loads, and put a big load on the generator head. On questioning Tom, it seems he was aware of this pathetic performance yet having no other solution, offers it. His AVR does work fine on the harmonic, and is a good value for that application. I suspect it might actually work if configured on the 240V mains output (balanced), but he has zero documentation for it and I don't want to fry my neighbor's unit experimenting. In my own system, I have a 400 watt toroidal step down transformer to 120VAC which provides excitation from the mains. This is not as good as the harmonic in starting motors, but it will start my washer (1/2 hp) when the well pump (1/2 hp submersible) is running, so is adequate for my needs.