The first step, I would think, is to know what's really happening with your coolant temperature -- an independent measure. That would be with a IR (remote reading Infrared) thermometer -- they're available all over, and they've become really cheap (given their value) nowadays. I like to aim at (even hold it near) the thermostat housing cover for the most accurate reading.

If the fluctuations occur while you're driving, just pull over off to the side of the road, pop open the hood and take an instant reading -- compare that to your temperature gauge reading. Otherwise, if it fluctuates while you're just idling, just watch the gauge and take the IR readings when the gauge acts up.

Even though the dash gauge isn't calibrated in numbers, you'll know if it's right by making some readings when the gauge is reading high, low or in-between. That will give you a sense of what's happening, at least a place to start.

After all, if you don't know the true coolant temperature, you don't know whether the problem is really your dash gauge (or its sender in the block, or wiring in-between), or is really a bad thermostat or (maybe) radiator core or rusted/blocked coolant passages in the engine, or whatever.

You've put a lot of money and effort into that list of replacement parts so far, but it hasn't worked yet, so you really need to find out whether its really a coolant cooling problem or something else.