Does anyone actually want iTunes? As a Spotify user, my Mac opens iTunes when I press the Play button on my keyboard if Spotify has been not playing for X minutes (power nap?). I have to immediately kill iTunes before it begins playing a CD I bought 7 years ago.
beardedspice might help with your media key issue
https://github.com/beardedspice/beardedspice
or maybe you can force the key assignment with karabiner-elements