I'm not against trying JRemote, however when I download music to the phone, I don't want it to be limited to it working correctly with only a single app. In particular I do hope & expect it to work correctly (including album art) with the default Android music app.
So I don't see JRemote as the solution to this issue. I would like to figure out whether there is a way to MC to handle album art in a way that is compatible with the default player, and I appreciate the help toward getting that to work. I may check JRemote out though - thanks for the upvote on that :-).
When I look at an album of music tracks (FLAC) for which album art is not displayed correctly in the default Android app after Syncing using MC, I do see that the tag says "inside file (300x300 - 12.9 KB)". So apparently it's not due to that alone.
I do see that the ""Enable album artwork support if possible" option is present if I select a USB hard drive, but is not present, when I select the S9+. That doesn't seem right (imho), and it does seems likely that's at least part of the issue that I'm having. So for now I'm trying another tool (mp3tag, MediaMonkey, etc.) to embed the album art in the file/tag if it is not already).
Using this method (another app + MC), I've been able to get a couple of albums to correctly display album art in the default (Play Music) app on the phone after being transferred using MC, however I've discovered an Album where the cover art seems to be "inside the file", but this still doesn't fix the issue - even after deleting the thumbs DB in the Android internal storage, and forcing it to be rebuilt.
I'm going to see if I can find an mp3 editor for android which allows me to actually see the tag values (as opposed to automatically updating them) to see if I can spot what's missing. It might be a couple days though - I've spent WAY more time trying to get this to work than planned, and need to focus on some other things for a bit. I am curious (and a bit stubborn :-) though, so I'll likely come back to this at some point to try to figure out what's going on. If I do, I'll post what I learn here.
(Btw just fyi in case anyone else tries to use MC to transfer files in a similar scenario, I also noticed that MC defaults to a slightly different folder structure than I had on in the music folder on this phone. That's not a problem of course, but until I noticed that, I had a duplicate copy of the albums I was testing with, which confused the issued the issue until I removed the original/duplicate files).
Thanks again for the suggestions.