How do I consistently transfer MP3 and M4A music files with cover art from Windows 11 to an iPhone 12 using Apple Music?

Before Apple music I used to use iTunes to purchase albums and add them to my library. Now I have an iPhone 12 I am trying to transfer the music files (RIP'd from my older CD collection) on to my iPhone. But it seems very inconsistent. I put the music Files in: c:\users\'ME'\Music\Apple Music\Media\Music (as that is where the files seem to go along with where iCloud would expect them no-doubt). On my Win 11 PC I have Apple Music, iDrive, Apple Device, iCloud and iTunes (probably don't need iTunes but it provides a RIPer function). With Apple Music I ADD folder of the album from the above Music folder. Then using Apple Device with iPhone 12 plugged into PC - I use SYNC. Sometimes they transferred and sometimes not. Sometimes the cover ArtWork transfers and sometimes not. I thought maybe rather than using MP3 which I have always used on my Android phone and using VLC - I would put the files in M4a(Apple format). AND I also embedded the cover art work at 1300, 1300 pixels as an article on the Internet seemed to imply they should be of this size or larger. (Or it said older devices used 900 x 900 pixels). I have always added and edited the TAGs in the music file. Using my music library App Bliss. It mentions TPE2 in ID3 format - is this something I am getting wrong? Also it mentions Apple Tags com.apple.itunes.coverart and others maybe I need to edit ensure these are filled in? Maybe and experienced knowledable Apple user can advise me not just how to transfer but how to get the music to transfer cleanly by SYNC. Maybe explain all the TAGs that important to Apple Music Files and Apple Music as a music player on iPhone as an APP. I have found this all very frustrating as an ex Android phone user. Thanks for all this forum does. Looking forward to a detailed answer about transferring files including any details that are needed for clean Sync. Thanks for listening.

[Re-Titled by Moderator]