Just in case you are dealing with a similar issue. I made a lot of progress and have everything set up to my satisfaction. In essence:
I no longer use the CD Rip Functionality from the Naim player. Here is what it does: (1) it rips the CD and converts the files to FLAC files (2) it looks up tag data and artwork in an online database (3) it puts everything on an external USB hard drive in what NAIM calls a ‘Music Store’ with each CD in a separate folder (4) in each folder it adds a file called meta.naim which has data about the files in the folder. This is all well designed and it works, but for a perfectionist it isn’t great. If there is no artwork in the external database nothing shows up of course. Also if you connect the hard drive with your computer and you look at the tags inside the FLAC files (with Tag Editor for Mac) you can see that it still fairly messy. But you can’t use Tag Editor on your Mac to clean it all up, because then you cause consistency problems with the meta.naim files. Lastly, FLAC file tags can be edited, but adding artworks is patchy (at least with Tag Editor).
INSTEAD - and this works like a breeze: On my Mac I made a properly curated set of folders (artist/album/[file1],[file2],[file3]) with M4A music files inside the respective folders. The M4A files tags I edited with Tag Editor, and I added the correct Artwork to each and every file. To the extent I needed to convert files from FLAC to M4A I used “To M4A converter” from the Apple App Store. In ‘Format’ inside “To M4A converter” I selected “ALAC” with Bit Depth, Sample Rate, Channels all “As Source” and Compression “Low”. This produces files that are nearly exactly as large as the FLAC file. “To M4A converter” can also be used to rip CDs, in preferences under ‘Tags’ you can actually select and option to launch Tag Editor after conversion, this makes the process of ripping CDs and then cleaning up the tags very efficient. The resulting folder with subfolders and files I put on a hard disk with Solid State Drive, connected it to the back of the Unity Star (I used the USB slot where the drive with the ‘Music Store’ used to be). It takes a couple of minutes but the NAIM then showed and played everything perfectly. As far as I can see NAIM doesn’t make any meta files in my folders and if I want to add or amend I just pull out the hard drive, make the changes on my computer, stick the hard drive back into NAIM, let the machine reload everything and ready!
As a bonus, my neatly organised folder with subfolders can be imported straight into Apple Music (the successor to iTunes), by just copying it all into the Media folder from Apple Music and then adding files. Apple Music will now essentially show the albums in the same way as NAIM does.
Lastly, my NAIM is also set up to play music that I have on my iPhone or Computer.
So, in the end: I will forget about the whole FLAC thing. Instead, my HiRes music will be ALAC that sits inside my Apple Music Library. A selection of my Apple Music Media folders I will copy onto an external hard drive that I will connect to the USB plug in the back of my NAIM.
In that way I curate music files only once. And my NAIM can play from its own external hard drive, or I can push music from my iPhone or computer to it.