Since I had some luck with cleaning on Friday thanks to the suggestions here I was confident enough to try cleaning the one high flow WPA I had. After the soak it seems like it was left with the same black buildup that would not soak or rinse out before but it was a lot easier to just swab out with a q-tip since there were no hard to reach corners. The buildup seems to be the same stuff that I get inside of the domed screen I use with the OEM stems, it’s just that before the Solo III it was only ever on the bowl side of the glass screen so it could easily be cleaned out. It seems to make sense that it gets on the other side with the III though since the III gets hotter, so I guess it’s melting it more to get down there. Anyway, it seems I can resume my normal cleaning routine with the high flow stems and will just have to clean the OEM ones a lot more frequently.
Also used one of the OEM stems that I couldn’t get clean. A little into a cycle, empty @ 428F there was some condensation in the stem but once I swabbed it out no more appeared for the rest of the cycle and everything seemed normal actually using the stem.