Okay I got another stupid question. You have everything going to the rear of the pc, but often times motherboards will have a riser to send audio to the front or top of the case so you can plug your headset in there. Do you have this facility and if so does running to it make any difference?
I may end up having to bow out but if I don’t get to keep trying to help: at some point you’ll need to fire up a daw or obs or jack or something to figure out if you can actually see the signal you’re dealing with anywhere.
The troubleshooting process I’m working through is more akin to what you’d do if you were at a big old mixing console trying to figure out why there’s no sound as opposed to the seemingly more obvious process of tracing device drivers and whatnot.
It’s been very helpful to me when troubleshooting sound issues “in the box”, so if you get stumped fiddlefarting around with lspci and whatnot, give it a shot from that side.
If you can accept 11” the old intel macbook airs are what you’re asking for.
They’re in the free - $50 price range now.