Hi Steven,
The 'Import' and 'Export' functions have been fixed since the 'B2B Jul 1 2020' release so if you ensure that your B2 software is at least up to that date or newer, then your upload will be good. Those work-arounds you mention are no longer required.
You can check the B2's software version using the B2 remote; press the 'Info' button twice and look for 'Version .....' on the B2 display.
On your USB device, if you create a top folder called 'b2Export', then in this folder create a folder called 'music', the place all of your music folders in there, you should have a trouble free import. As Edwyn states above just ensure that you use the folder structure that he has described.
If you use 'Import' then your upload should take about 16 hours by my estimate.
The SPDIF optical out is the same socket as 'Line Out' on my B2.
Good luck!
Regards,
Peter.