Formatting an External SSD for Mac
Apple recommends using APFS for SSDs. You cannot use APFS with a Time Machine Backup Drive. If you are planning on using it for two things, storing files, and Time Machine, then I would recommend partitioning the SSD and then making the one for storing your music, etc. APFS and to use Time Machine, simply go into Time Machine Preferences and then select the partition meant for Time Machine. Time Machine will automatically format the partition as HFS (macOS Extended).