Hey there! OP here. Since this thread has kinda blown up, might as well post an update:
I never did get this to work. I ultimately traded that phone in for an iPhone 12 Pro Max.
This new phone, when it arrived, seemed to work just fine. Plugged it in to my main computer (a Linux box), and it recognized the cable that, for the last two months, would not work on the iPhone XS Max. It worked on my Windows laptop, which I plugged it into so I could charge it while I was attending class.
Immediately after this, attempting to plug this *brand new* iPhone 12 Pro Max back into my Linux machine, the phone would not recognize it, and refused to charge. The exact same set of symptoms as was on the iPhone XS Max.
Now, this phone *does* connect to other items. It connects just fine to the aforementioned laptop at this time, and the cable was confirmed good by charging the phone with it from the laptop. It connects just fine to my car. It connects just fine to my work computer, another Windows machine. It connects to a Mac mini of mine, something else the iPhone XS Max refused to do.
It just doesn't seem to want to see the Linux computer.
It's as though something in software has created a connection lock on the phone, which matches pretty closely to what I'd seen on the iPhone XS Max (though significantly less universal). My best guess is that, by plugging it into my Windows laptop, iTunes queried the phone, which my Linux machine did not do (for obvious reasons). So then after that, even though the Linux machine has a perfectly viable USB signal and it should at least be *charging* on that computer, it's refusing to do anything.
Similar to the iPhone XS Max, this phone clearly sees that USB is being offered; it will not charge on a wireless charging pad if the USB cable is plugged into that computer. (This is, again, exactly the same symptom I saw on the XS Max.) And, as my wireless charger works when plugged into this exact same computer, to charge this exact same phone (which is how I was charging the XS Max previously), that clearly demonstrates that the USB on that computer is absolutely functional.
So. Cable's been tested. USB has been tested. And the charging circuitry of the iPhone 12 Pro Max has been tested. And all are clearly demonstrated as working correctly.
But, when this phone is connected to that specific computer, it refuses to acknowledge the USB.
I think this is pretty strong evidence that there's something *seriously* broken in iOS 14.
