Adding iPhone music library to iTunes on Mac?

Hi! I have a 1st generation iPhone SE and an older Mac running iTunes on Sierra. I prefer to have my own music library (purchased from iTunes or uploaded from CD). I currently have different songs on my iPhone and my Mac iTunes. What's the best way to sync my libraries so I don't lose anything, and so I just have one library I can easily add to a new device? I'm considering upgrading my phone and don't want to lose all of my music. SOS!