It seems to me that, everything else operating properly, the thermostat will determine at what temperature your car will stabilize. Since you changed the thermostat, you may have gotten one with a lower temperature specification, thus the lower running temperature.

My 1982 300D tends to run about two "needle widths" below the 100 degree mark most of the time. During hot weather with the AC on it will exceed that a bit. So, I don't think you did any damage to the engine by running at 100 degrees, if everything else in your cooling system was operating OK.

I'd probably like to run a little cooler, like yours is doing. It especially sounds good that there is no change during harder conditions. Flushing your radiator probably did you some good.

Where did you get the thermostat, and do you remember if it had a temperature rating?

Will be interesting to hear from others where their 617 engines run on temperature. Also, I would guess that the in-dash temperature indicator isn't exactly instrument-quality in accuracy, so there might be some variation among cars depending on variations in the temperature sensor, the gauge, and the interconnecting wire condition.

Ken