Ratio of two RMS values in recorded audio
The difference is their ratio and this can be shown by converting from dB to an equivalent but arbitrary voltage.


-15dB converts to 177.8mV
-7dB converts to 446.7 mV


The ratio in these units is 2.512 and reconverting back to dB is 8dB

The math you did here: -


  20log(-7/-15)


Is not applicable to the decibel ratio at all and is basically nonesense.

The conclusion here: -


  I reckon That means that the dynamic range in the chorus has collapsed
  by 6.62dB, is that correct?


Is based on a meaningless premise i.e. the formula you used above.

Measuring dynamic range requires looking at the RMS and also looking at the peak value, and, in the case of a modern piece of digitized music, the peak will be somewhere close to 0dB.

BTW I also produce music!