So my device is an RZ03-0038 (not 0038x, which seems to be the 2014 version). Mine has 3 level (plus breathe mode) blue LED lighting, not the 2014 green, or chroma lighting. Mine also conveniently has a macOS layout which seems to have been discontinued not too long after 2013. I think mine was only ever known as "Razer Black Widow Ultimate", and then the later models had other designations (like 2014 or Chroma) but those designations weren't always used in publications because newer models superseded older models. All of which is to say that YMMV, as things might have changed slightly on the firmware for later keyboards - I don't know to be sure.
I have the "Use F1, F2, etc. keys as standard function keys" in keyboard settings checked (enabled) and the top row of keys currently works to do their labeled accessory functions (i.e. play pause music) with a single press. However, the keys on the left hand side (F15-F19), as well as F14 at the top right all report their respective Fxx value to the operating system and can be used in Keyboard Maestro as F15 etc.
The top row keys can be used as F10, F11 etc by combining with the "fn" key beside the "home" key. It now occurs to me that this is possibly quite different behaviour if you don't actually have a macOS layout Razer keyboard....
It is weird that the top row does their accessory functions, while the macro keys do their Fxx functions, but having never really used the top row for macros when I was using this keyboard a lot it didn’t bother me.
I've now disabled the "Use F1, F2, etc. keys as standard function keys" checkbox in settings and can report even weirder behaviour. For what it's worth, I usually have it enabled because I now use an Apple Magic keyboard and I like to use the top row for my macros as it doesn’t have dedicated macro buttons. Anyway, with the checkbox disabled my Razer keyboard still reports macro keys identically to with it enabled, but the top row only does the accessory function regardless of whether or not I press the fn key.
Finally, if this is at all useful and you'd like me to try different keyboards I do have two other Razer keyboards. An identical model to this one, but with the windows layout, and a ten keyless "Tournament Edition" from around 2015. Happy to try those, but I'll need to get them out of their boxes (wherever they are) to do so.