How to sync certain songs only to certain devices (iPod classic and iPhone with Apple Music)

I guess my question more concisely is on how to to prevent songs downloaded for my iPod from syncing to my phone, so that I only have the Apple Music versions of tracks on my phone. I would prefer not to go through my duplicates and remove the Apple Music versions, because the quality and release versions would inevitably be inconsistent. I want to keep both versions of all the duplicates, but prevent those tracks of non-Apple Music-standard file types (the ones meant to sync to the iPod) from syncing to my phone.

I'm thinking the initial way to do this would be to never have synced my phone to iTunes in the first place, but I'm far past that. So, alternatively... how can I remove these tracks from my phone? There are thousands of them, so I would need a way to remove them en masse rather than selecting them one by one. Hence my line of thinking to set certain songs to not sync in iTunes. I suppose in this case I would have to sacrifice wanting to sync certain tracks to both devices (ones that are not available from Apple Music, e.g. demos or unreleased tracks). But if there is in fact a way to keep those on my phone while removing others, that would be my preferred solution.

I forgot to clarify that I do not have this issue with duplicates appearing on my iPod, because all the Apple Music versions of tracks won't sync to it because they are not compatible file types. The duplicates only appear on my phone.

So how can I make it so everything in the smart playlist does not sync to my phone when I plug it in? Sorry if it's obvious, I don't really know what all the functions of a smart playlist are because I haven't followed iTunes updates for a few years. All I can tell smart playlists will do is auto-sort songs for you based on your "rules," which I set my sorting rule to be tracks with "kind" containing "MPEG." Unless there's something I'm missing, I can't figure out how this would prevent them from syncing to my phone since all of the songs in the smart playlist are also still listed in the general library, the entirety of which syncs to my phone when I connect it.

Honestly a regular playlist with everything I didn't want to sync to my phone would work best if I could mark it to not sync to my phone, because there are still MPEGs that I do want to sync to my phone, so applying a rule like that would exclude those.

Is there still a function like, "sync only checked items" like there used to be? If there were it wouldn't be a problem for me to check and uncheck songs (so long as i could select many at a time and "check/uncheck all selected") depending on what device I was plugging in. But as far as I can tell those "check" boxes are no longer a function of iTunes.

But how, specifically and step by step, do I “setup the iPhone to sync with A & D, and the iPod classic to sync with C”? I am not having trouble identifying the tracks. I have already contained them in playlists like you described. I just don’t know how you set certain playlists to sync and others not to. And how you adjust those settings depending on the device.

I know checkboxes are a global property. But if I can check and uncheck many at a time I don’t mind doing it every time I want to sync a device. How can I access this function again in the current version of iTunes?

I have a Mac.

Forgive me if I switch tones here but this is driving me absolutely insane. This might be becoming another issue entirely, or several issues, so I'm just going to try to explain what's going on.

I followed your link above. I already understand how to do everything that article mentions. Unfortunately none of it really answered my question. So instead I went into iTunes (or "Music" as it's vaguely named now) and fiddled with some settings. I made it to where I can now see those checkboxes, so I unchecked all the songs that I did not want to sync to my phone, and selected "sync only checked songs" on my sync settings for my phone in iTunes. Then I synced it. But everything remained the same. Nothing was removed.

So I searched on the internet how to mass-delete music from your phone without deleting it from your library. I learned that on my phone, if I go to Settings > General > iPhone Storage > Music, I can view all music downloaded to my phone sorted by artist. I decided that was the best way to mass delete songs from my phone I could find, considering the only other alternative is to delete each song from your phone one-by-one (not an option for me, because I have almost 1400 songs I want to remove). So in iTunes, I began to rename the artists for the songs I wanted to remove (I contained them all in a playlist). For example, I changed the artist name "King Krule" to "ipod - King Krule." A little tedious, but I have a lot of songs by each artist so it only took about 30 minutes -- significantly less than going track-by-track. Then, on my phone, I went into Settings > General > iPhone Storage > Music again and saw the new artists with "ipod" written in front of them. I swiped to delete each of them. My plan in doing this was, from now on, to only listen to Downloaded Music in Apple Music, since for some reason the unchecked songs would not disappear from the main page of Apple Music (as I said in the last paragraph).

However, I now have a new problem I can't wrap my head around. In my Downloaded Music section of Apple Music, albums are incomplete. As an example, the album Nonagon Infinity by King Gizzard & the Lizard Wizard only lists tracks 3, 4, 5, 7, and 9. So I looked up this album on Apple Music and pressed the iCloud download button for the album. But none of the missing songs are appearing in my Downloaded Music. I tried again, this time clicking the iCloud download button for each track that was missing on that album. No luck. It's still incomplete, and so are many other albums.

Both my Mac and my phone are fully updated. The Apple Music app on my phone and the Music app on my Mac are both fully updated. I tried turning my phone off and on again several times, and tried to download these missing songs again. Still not there. I hope I don't sound rude, I'm just losing my mind over something that I thought would be simple (mass-removing songs from my phone but keeping them in iTunes). But in the process of trying to troubleshoot that my problem has multiplied. I'm going to try to contact someone at Apple about all this and send this thread along to them. So if you don't have an explanation or advice for all this, don't worry about it. I appreciate you trying to help so far.