> The most important question was the structured cabling in the walls; was it CAT-5E or CAT-6, or even CAT-6A? Remember from the last post, 10GBASE-T might work over short runs of -5E (even though officially it's not meant to be able to).

This is not quite correct.

The primary problem is cross-talk. Copper wire itself will carry the relevant frequencies up to 100m without issue but even with balanced pairs the balancing is not perfect and the "dirty paper precoding" is not perfect so some cross-talk will occur. How long you can go with Cat-5e depends on how well the wire is twisted, how many wires are bundled together, are there any loops or tight bends, and other factors. Cat-6A guarantees less cross-talk with more twists, better balancing, and a plastic separator inside the cable to make the cross-talk more regular and thus easier to cancel out.

Bottom line is: for almost any normal home or apartment any quality Cat-5e cable properly terminated will carry 10GBase-T without issue. In fact if you have problems I would first re-terminate the cable before assuming you need to run new cable. Cat-6 or 6A just isn't necessary.

As a PSA: beware of "CCA". I've noticed Amazon and eBay are absolutely flooded with cheap chinese electrical and networking cable that shows nice shiny copper in the pictures but is actually "copper clad aluminum". If they mention anything at all they code it as "CCA" cable without explaining what that means.

CCA cable cannot, by definition, be ethernet cable. I won't get into the full technical details but the standard was amended to clarify that only pure copper wires are acceptable for ethernet. Personally I would not dare use CCA for anything. It has lower performance, lower current-carrying capability for the same wire diameter (inherent in aluminum), and introduces the risk of oxidation and loosening of connections as people will treat them as copper connections when aluminum needs special installation procedures and connections to avoid them coming loose over time. For electrical connections especially this not only can but absolutely will lead to a fire over time if not treated with the appropriate care. All it takes is a little bit of mechanical action scraping off the thin copper layer and you now have an effectively aluminum wire - a time bomb ticking away.