which it hadn't occurred to me to check as part of the cooling system. It's funny how some of these things seem obvious only after someone points them out to you!

That hose is indeed pretty small in diameter, and the fitting on the front of the thermostat is pretty small too, and I can see how it being partially restricted would restrict flow on the freshwater side. Hadn't realized it's steel and not bronze...so rust over time could certainly clog it up.

Today I'll check my raw water flow at various places (djmarchand's advice here) and if that checks out OK, it's time to look at the freshwater side of things...thanks for the tip.