I am just pondering this issue, and I think it’s important not to condemn anything too quickly
My 3.2 (with multi layer steel head gaskets) has developed a brief miss from cold this winter which clears very quickly (sometimes after a restart). It also loses a teeeny amount of coolant (250ml from here to Gibraltar and back plus a few months running around). I know that’s not HG, it’s just a minor annoyance. But I think given the size of the exhaust and how it behaves in the cold (appearance of mist) some mechanics may misdiagnose.
Your change of coolant colour would be more likely oil cooler than HG, the layout of the waterways dictates this. But if it’s just brown without sludgy deposits that’s also inconclusive. I’d also expect excess pressure if the coolant is being infiltrated by combustion gasses etc.
You may not be aware but I think I live less than 15 miles from you these days. Let’s have a look at it one weekend

Long shot, but do you have links to any threads etc on here surrounding the rebuild of this engine? I may be getting muddled, but do think I vaguely recall reading about it before and recall there were perhaps a few small modifications?