Rainy 75: How to remap F-keys with FN layer?

Hey everyone,

I recently got a Rainy 75 Pro and I'm really enjoying the board so far. Now I want to customize the layout a bit (ISO-DE layout user) and I'm running into a question regarding the F-Keys and the FN-Button.

What I'm trying to achieve:
I want to assign custom functions or special characters to the F-Keys to use with FN (for example FN + F1 = volume control, macros, etc.), while keeping the normal F1–F12 behavior intact.

I found the JSON file on the Wobkey support page (Link: https://www.wobkey.com/pages/rainy\_75\_support) , but I don't really understand how to modify it or how to add custom functions or FN-bindings. Please help!