Artwork Issue When Using a PC to Load MP3 Files onto an iPhone

I have hundreds of albums in FLAC format stored on a NAS connected to a Windows 11 PC. I am converting some of them to MP3 files to load onto my iPhone 15 Pro Max running the latest iOS.

I have downloaded Apple Music and Apple Devices apps onto my PC. My FLAC files have tags including album artwork that I manage with Mp3tag. I am using J River to convert files from FLAC to MP3. The resultant MP3 files contain tags and artwork.

When I copy (drag and drop) the new files into Apple Music there is no artwork showing in the app. I just see the generic "note" icon:

But when I click properties on the album I see the artwork:

I tried clicking Add Artwork and chose the folder.jpeg I keep in the album folder and then I see the artwork in Apple Music:

But when I sync Apple Music with my iPhone using the Apple Devices app there is no artwork in Apple Music on the iPhone:

I see no way of adding artwork in the app on my phone. Why is this so difficult? Can anyone help with this?

I would love to be able to convert and copy without a bunch of manual steps.

Thanks,

Martin