Here are some possible next steps. Whether they're worthwhile depends on your needs.
I've done this on a separate computer. There are some caveats:
This is an over-simplification - see the elm-chan doc for more details. But in many circumstances, the SD card will continue to drive MISO even when CS is high (i.e. deselected). This interferes with other devices that might be sharing that data pin.
An easy but hacky solution for this is to connect the SD card's MISO input through a resistor, e.g. 1k. Depending on the nature of the other devices, this may be enough impedance to let the other devices work properly. Otherwise you need to either not share this pin, or use a transceiver, or try the mitigation from the elm-chan doc (which is basically to pulse the clock 8 times after CS goes high at the end of a command).
On startup the SD card is not operating as a true SPI device - it is using its native 4-channel protocol, and in this state it reacts to inputs even when CS is high, contrary to usual expectations. It's only after the initialization sequence that it stops reacting.
One way to mitigate this is to ensure that whenever CS is high, MOSI is also pulled high. In this state, even if the clock pulses, the SD card won't mind. You can do this by adding a diode from CS to MOSI near the SD module, and a resistor from MOSI to the 6522.
The code to read and write bits struggles a bit because the data to read or write must be held in the accumulator (in order to shift it) and the PORTA values also need to be in the accumulator, so we can use ORA/EOR/etc to toggle SCK.
However, if we move MISO to PB7 then the negative flag will get set based on the MISO value when we read from PORTB into any register, and we can branch on that straight away without needing any comparisons. So we can let A store the result permanently, without needing to swap it in and out.
And if we move MOSI to PB7, but keep CS and SCK on port A, then we can very easily write the whole output value to port B, shift it, write again, etc.
TBC