I do have Karabiner installed. It does make most of the media keys to work with both keyboards actually. F3, F4, F10, F11 and F12 work fine.
F7, F8 and F9 don't, on either the WASD or Ducky.
It feels like it's a system thing, not a keyboard thing.
Do you have these media functions working in Mojave?
I searched and found a bunch of similar reports.
What I did: completely uninstall Google Play Music extension on Chrome which could be preventing the keyboard shortcut to bubble up in the system. Also made sure no global shortcut exists in Chrome right now.
After that, pressing F8 would launch iTunes. Not sure what I did, but now it doesn't.
Attaching my Karabiner configs. Also, Karabiner's event viewer app seems to understand the media functions, but for some reason it doesn't make Spotify do anything.