iTunes/music app issues syncs

Ok so I have a Mac Mini 2010 always had iPhones, I have about 200Gb of music stored on a hard drive that is the source on iTunes app for my music.

my collection consist of iTunes purchases, my CD collection imported in and various other downloads over the many years.

my issues happening when I do manual managed syncs with the phone and my Mac I had a playlist for example of Hip Hop songs 1440 when I sync tonight it has removed songs and gone down to 1040 and I have no idea why it’s doing this ?

I do purchase music via iTunes Store in my phone and on the Mac iTunes/music store and everything is sourced and saved to my external hard drive I don’t use the Mac mini hardrive at all and nothing is stored in it music wise.

I find iTunes super annoying and this has become a new annoyance when I’m trying to update playlists with added purchases or downloads that I want to add to curate a new version of my playlists as I am always buying and downloading music.

my Mac and iPhone all authorised and my Apple ID is signed in on my iTunes in my Mac mini.

can anyone help ?

is there any other programs other than music app to use for music in iPhone and Mac but still have the ability to add music to and from ??

im getting sick of curating all the time and rebuilding playlists that i edit I do not understand why its removed songs when i have tried to make the playlist the same or with more songs that it essentially should just add to the phone it’s bloody stupid.