Sync issues catalina

everytime I sync entire 10000 song library syncs in full. I have 450 cd’s ripped included. Any cd ripped since Catalina doesn’t do the repeated full library sync. I can drag and drop as workaround. I do not have Apple Music or match. Cloud of any kind Is OFF. Never used. So two problems entire library syncs everytime and Catalina ripped cd’s go into iMac library fine but must be dragged and dropped manually to phone. Restored phone as new. Went through every setting with Apple help. Only recommendation. Get Apple Music! Reverting previous OS very difficult as wife needs and uses many updated apps etc that don’t work on older OS’. I live in fear manual setting will revert to auto and the second phone is plugged in I lose all and wait 5 hours again