I don't know if anyone is still interested, but I think I somewhat understand part of why I have been experiencing discomfort: the third valve tubing positioning on my horn. I have a jp174IL, which has the 4 valves inline, and I realized the way things are situated, the third valve tubing is parallel with the main body of the horn and also a bit further out, making a more awkward angle to wrap an arm/wrist around.

I thought maybe I'd just been away from the instrument for so long I forgot how uncomfortable it was, but no, I just spent most of my time playing (primarily school owned) yamaha and willson compensating horns which have their 3rd valve tubing at an angle/more flush with the rest of the body.