Wouldn't it work if CS8421 is set to slave input/output, input set to RJ16bit, output to 16bit i2s and then to WM8805 HW mode, master set to 16bit i2s input (MCKL output would be 256fs=11,2896MHz)?
Input and output in slave mode, cd player is master to the input and transceiver to the output. The module boards on aliexpress are cheap. If you don't succeed it will not be financial disaster.
It would be nice if @amirm could do freq. resp. measurements vs level. @napilopez used to do that and it nicely showed effect of dynamic eq for speakers with limited bass output.
It is dynamic eq - any eq that changes fr over level is dynamic eq.
Yes, "bt speaker" (as you call it) uses the same limiting technique to prevent driver damage.