a recent report put my husband’s reporting of my PEM down as “lethargy”



Sorry, it's not actually funny, I know, only when I had my first spell of ME as a child I wasn't diagnosed with ME but with "fainting and lethargy" (and promised I'd grow out of it...)
Anyway, re
@Andy's question:
During my 2 near complete remissions I got a very mild form of PEM that I mistook for 'some virus' at the time (I wasn't diagnosed back then).
It always started the day after some bigger than usual exertion; I just assumed I was a bit run down because of the exertion and therefore more susceptible to catching 'some virus'. Though I could never work out why I seemed so prone to becoming 'run down' when everybody else seemed to be able to just shake off the same level of exertion and do it all again the next day. Whereas I was having to take it easy for a few days, waiting for that 'virus' to disappear.
Symptoms: first there was a sore throat soon followed by that run-down, under-the-weather, slightly feverish, slightly achy, brain-not-quite-with-it feeling you get when you're coming down with a bad cold. Except the expected runny nose and cough never eventuated.
I was also always aware that I was more easily fatigued, physically and mentally, than others around me, not hugely so, but enough to be obvious, and for no good reason as I seemed otherwise very fit and healthy.
So, a lack of stamina, delayed flu-like symptoms and brain fog - all mild - were the PEM features I got even during near complete remissions. They're still my main PEM symptoms now, only much much worse. So even when the ME was at its mildest my PEM symptoms fitted the CCC/ICC PEM/PENE descriptions quite well except for their lack of severity.
I also had various other mild symptoms, notably OI, right through but I can't recall if they worsened during PEM then, though they certainly do now.
ETA: A lot of people here seem to report versions of the "virus" that wasn't a virus. Maybe that's something to work with?