With those symptoms I'd first check the fan clutch. If the air coming out from around the fan shroud is good and warm but the fan is mostly coasting on the shaft, it's bad. Could also be general crud in the cooling system but seems unlikely with your regular coolant flushes.

That's about the most gentle way to overheat an engine - got warm slow, didn't shut off and heat soak or rapidly cool, and got cooled down gradually and completely. If any damage was done it's only because said component was on the brink anyway.

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'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".