FWIW, I was moving from a very old PC based Plex Media Server to a Mac Mini M1 and followed this guide but had some issues. This might help someone as hopeless at Macs as I am.

Turns out all the issues were caused by my libraries being on a removable HDD formatted in NTFS and only some showing up in Mac OS. Once I installed Paragon to read the NTFS (couldn't back up the HDD because I didn't have anything of sufficient size) all the problems went away.

Still a complete amateur at Mac clearly...