Chasing this cooling issue for months

That fan is not effective. Fans MUST HAVE a full radiator shroud. If you look at ANY factory electric fan setup, they have a shroud covering the entire core. Even if the fan is pulling "enough air" it is only using a small part of the rad.

Earlier you mentioned that "if the gauge is wrong" it means that the engine is running too hot (230) at low speed. This may very well be---a cooling system that is leak tight and with a good cap CAN run at 230, and without an accurate gauge, you would never know it.

I would get something else to check your engine temp.