Steve, have you tried something like the TP-Link optical media converters or maybe an optical isolator in place of the modded switch? (Presumably powered by a clean power source on the receiving side, at least.)
And if this is beyond what you want to divulge, I understand, but does your specific Ethernet input to the Interchange minimize the impacts from these upstream devices? Standards compliant Ethernet should have a transformer on every jack intended to provide galvanic isolation, ground offset mitigation, and common mode noise rejection. I'd normally assume that such an interface would be less sensitive to the benefits provided by tweaked switches, etc.. Like most systems, however, I assume that the quality of implementation will vary, and with it, the potential benefit from such upstream component choices. I'm guessing that off-the-shelf transformers could be improved, for example.
Your implementations get rave reviews, and I've enjoyed every RMAF demonstration of yours that I've heard, but I guess I'm curious as to why some things matter, mostly as an aid toward integrating the system as a whole.
FYI, another Roon/HQ Player user here, so I'll keep watching for any news on that front. Thanks!
(By the way, while I still use HQP for upsampling and driver (not room) correction via convolution of an impulse response filter, Roon bears continued monitoring relative to their sound quality... their overall upsampling/convolution/DSP capabilities and implementation have been improving with each new release.)