I can’t speak for others; speaking about my machine, only the coolant is getting hot the hydraulic fluid is staying around 125 to 130°.Cavitation is caused by air in the system, lower viscosity fluid won't help that.
Is it safe to assume the machine quick connects are sized correctly? I only ask because we had issues with a 3ft road mill on our kubota that we didn't have on the old new Holland. Turned out the little fittings on the Kubota weren't actually rated for the high flow option. They caused a restriction in the system and heated up the hydraulics pretty fast. We also had (dealer) to add a case drain line that went directly back to the tank.
I've been following along, but am still at a bit of a loss if it's the engine over heating, they hydraulics, or both?