You could check ECT sensor at manifold compared to a IR thermometer aimed at it, temp senors work good/last a long time but not known for exacting accuracy. Possible sensor output high.

If you still have a overheating issue after all you've changed and system is bled you may want to pull the block drain plugs and flush your block well.
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One month later update/conclusion:

Along with the perpetual pressurized overflow and random overheating, I started hearing what sounded like an exhaust leak from the passenger side that just got louder. I threw up my hands and tore it down to the shortblock, had the heads tested/refreshed, and put it all back together.

The gasket was dissolved between 4 & 6, which answered the exhaust leak sound. There was some gasket corrosion between the cylinder and water path on 7, so I have crossed my fingers and prayed that was the source of the exhaust gas in the coolant.

In the end, despite all the Co2 testing, I think the only possible answer was that there was exhaust gas in coolant. I was very atypical - no coolant loss except when the overflow happened, and a negative reading on the Co2 test. No oil in water or water in oil. Weird.

I have also done the thermostat mod (live in west central florida - so no worries with cold) , and my temps are around 188. Still seems a bit high, and it fluctuates a lot, but the pressure isn't happening and all in all things are great. I'm going to assume this saga is over.

Thanks for all of your help.