I finally managed to get this work. Things I was doing wrong:
Not using the OPi.GPIO replacement library - others aren't compatible with the NanoPi M4;
DIN LCD pin was connected to the SBC RXD instead of TXD;
Passed wrong pin numbers to the libraries (should be the real physical pins numers);
Mistaking SPI port with device...
Here the full guide on how to it properly. Start by installing: