you can not pass a chip select pin. with a begin call. The ChipSelect pin is “hard” defined in the variant as ‘SS’ . it will work if you change
_spiPort->begin(_CS);
to
_spiPort->begin();
I hope that is enough as by doing a quick look at the code I see it is using an old style of interacting with the SPI library. The Arduino AVR library has some backward-compatible calls and that might be enough for this work.