ARHG!
Everyone needs to stop being an Audiophile with networks. Analog signal is far different over digital packets. The same rules do not apply! Noise is not part of ethernet, noise is not part of WiFi packet transfer. Yes, the radios are noise, but it is not part of the packet transfer, and in fact, it is air-gaped to the receiver.
You will NOT get any degradation in packets in runs under 200ft. NONE!
It is perfectly safe to run cat6 inside ducting. In-wall or outside cable would be "best" but not required. Use rubber grommets on the ducts, to run the cable through.
"clocking" for ethernet? NOPE, not a thing, there are no high end clocks in any enterprise networking gear. It is NOT the same as from the streamer to the DAC.
Having built in WiFi into your modem is not a bad thing, it will not put "noise" on your precious streamer packets.
If you are so worried about WiFi noise on your stereo, put it in a Faraday cage, even if you do not have WiFi in your home, it's still there, unless you live over a 1/2 mile to your closest neighbor. A microwave, puts out more radio noise over almost anything else in the house, as does your cel phone, and tablet.
Wired ethernet is almost better, be it fiber or copper. Fiber is the only transmission method that doesn't care about any kind of interference, or distance.
Almost all home mesh networks are garbage, talk about noise, lost packets, jitter, bad handoff, etc.. Run cable to a couple base stations, so much faster, and more reliable.