I though you have read my suggestion below and tried it!
"For spi loopback tests in Rpi python 3.5x and 3.7x, you can search this forum using key words "SPI", "loopback". If you add the search word "tlfong01" you might find a couple of my answers with loopback python test programs, colourful waveforms illustrating the CLK, MOIS, MISO, CSn waveforms. You don't need to understand the details of the programs. My programs are "plug and play/run" no library is required. Though you need to "uncomment" to select slight variation of loopback, say one, two, or three characters, or control characters such a <control x> etc. Happy testing, cheers!"