There is a big difference between 6..9 MHz clock speed (which is possible on a Pi) and 6..9 Mbit/s data rate which you won't achieve if you can only read 2-3 bytes at a time. The overhead of IO library and drivers will limit you to tens of thousands SPI transfers per second.
If you find an ADC that can be configured to buffer samples and them stream them over SPI in large chunks, it may just work with a direct SPI connection. However, ADCs which require you to read out the last sample before they can produce a new one will require external buffering.
Look at FT42XX products: those are USB to SPI/Quad SPI bridges supporting up to 30 Mbit/s clock rates. FTDI offers drivers for ARMv7 and ARMv8 as a part of their libft4222 package, so there's a good chance those bridges will work with a Pi.
Digikey sells FT4222H modules for $18 a piece.