I ordered the GPIO extension cable, prototyping hat with EEPROM, SD card breakout board, and SD flash chip breakout board from Adafruit recently. That's just enough to get me a single cart with an SD card socket connected over SDIO and a 512MB SD flash chip connected over SPI. The extension cable is needed so I can connect Pi HATs while my Pi 400 is in the Pidock. I wish I were more excited about all this, but I haven't had the time or energy. When the parts arrive, I'll solder everything together and start testing it out. Software will probably be the hardest part for me. It's my weak point so far. I think there are libraries out there for the Pi to use SD cards over both SPI and SDIO. If so, I'll install them and get started developing a very simple test program to read and write. From there it's just a matter of adding the features I want and creating an easy way of installing the software. I haven't touched Python in quite a while, so I'm sure it's going to take me a while to get it going. I've been playing with Arduino lately, so my C++ is decent enough. I was able to write an entire relay test program for the Raspberry Pi Pico, from memory,on the first try. That was a nice milestone. Let's hope Python comes back to me so easily. 

More updates soon after the new hardware arrives.