It would be even better if we could prompt the user to pair from the device plugin, when the user plugs the usb in for the first time.

I usually activate USB tethering on the phone side, and it "just works".

I have been thinking about implementing the KDE Connect protocol as the phone <-> PC communication protocol of choice for PostmarketOS when connected over USB.

But I second @mtijink on this: It would be nice to have a special mode for USB, where the phone prompts for pairing right after the connection (and maybe the PC as well, though there could be a "one-way control" until properly authorized). Having the phone appear in Doplhin as soon as it is connected would be nice as well (and maybe only prompt for pairing once the user clicks on it?)