It turns out that the post you linked was the ticket: one of the responses said that he had used a VM installation and that when he arrived at the same point I had, it was a simple matter of reconnecting the device. I had initially not investigated this solution because I had tried reconnecting many times, but I decided to give it a try because everything else had not worked, and it seems to have worked. My device is currently displaying "Installing update" with the UBPorts mascot and an orange dot indicating it is doing some work. I will be leaving it like that overnight just to ensure I do not bork any important operations.
For posterity, I will include a step-by-step description of what I did in case anyone else has this problem in the future. I feel pretty stupid right now, but I'm just so glad to have it working properly:
- Do the steps I described in my first post
- Get to the point in the GUI installer where it boots the device out of the Android bootloader and into the UBPorts Recovery mode (with the UBPorts mascot on the top right)
- You should have received a message from your Windows host machine that the device is unrecognizable. This does NOT mean you cannot connect it.
- Disconnect and reconnect the device to the Windows machine. If you have properly installed adb and the other preliminary files, then you should be able to connect it to the VM. In my case, I received a prompt from the VM because it recognized a device being connected.
- At this point, the GUI installer should recognize the device and its state, and begin downloading files.
- I did not experience any other issues to the end of the installation, and my device is now operational.
Thank you so much for your help, everyone. Now I just need to do this a couple more times so I have backup phones and I'll be set to check out of the Google/Apple mobile ecosystem.