Here is my feedback. As I didn't understand right away that I had to validate the remap with the 'set core remap' menu command, I went back to version 3.14 for a while: I realized that this volume shortcut was not even a setting, but natively implemented and immediately usable.
So your file does its job perfectly and I thank you for it.

Unfortunately with this development version I can't finalize a basic setting I commonly used:
backlight exemption doesn't work properly with the skip button for FMS preset stations and becomes downright inactive when using the lock function of the device (of course exempting the skip buttons). I'm not very comfortable with all these settings but it seems to me that this is a bug; for information, the daily release (20220611) behaves like the development version with this same inconvenience* which will surely be solved one day.
*EDIT :with these same settings, the 3.15 stable version doesn't suffer from this bug