@cHECKjAM
There's a few things confusing about that experience. An upgrade shouldn't run the first boot wizard, thus the pairing page shouldn't show up. Since it did on yours... yeah, I don't know what to say.
But the really worrying part is that I/O error. That's indicative of hardware problems. Do you have another usb cable you can try? If it's not an issue with the cable, then it could be issue with the emmc. I sincerely hope not.
Though, if you've already got 18.1 installed, you shouldn't need to manually flash another. I hesitate to suggest it, but you may need to do a data wipe and see if that boots as expected. But then you'd have to re-install everything which can be a pain.
@jmikepr
1) This is more of a user review question than one me the dev can answer. I use the official builds on my main setup, though.
2) I use gapps on my setup, so I again can't really answer this. There was a user a page or two back that said netflix worked without gapps, though. In general, it should work just a well with or without gapps, just missing the store for the latter.
3) This depends on the gapps package being installed. MTG is recommended by Lineage for 18.1. I've not tested the phone remote app. It *should* work, MTG installs the receiver app.
In regards to launcher, MTG does not replace the launcher. And Lineage ships a no-gms launcher that has the ui style from before google force-fed ads. So yes, no ads here.
4) Passthrough audio works, I use atmos on my setup regularly. Dolby vision does not work, Nvidia put this behind encryption and a locked bootloader check (probably forced to do so by Dolby).
4k in general works. The launcher and java apps are forced to 1920x1080 as required by Google. But media and native rendering will play at native resolution, up to 4k. The one exception is Netflix: 4k netflix causes horribly artifacts. So I had to disable that outright, making netflix fall back to 1080 so all content is accessible.