To add to my original comment, if you want to pursue the Wi-Fi SD card route, I suggest using any of the known vulnerabilities to get root on the card and then reverse engineer the card (as in how the SD side is driven) from the inside.
This would effectively let you skip the whole “build a device that emulates an SD card” part.
From there I’d suggest building a Linux image from scratch using Buildroot or Yocto, so you start with a fresh and modern base and don’t have to fight with the SD vendor’s firmware or deal with their vulnerabilities (which might be a liability in your case).
Feel free to get in touch (email in my profile) if you want more guidance.
This would effectively let you skip the whole “build a device that emulates an SD card” part.
From there I’d suggest building a Linux image from scratch using Buildroot or Yocto, so you start with a fresh and modern base and don’t have to fight with the SD vendor’s firmware or deal with their vulnerabilities (which might be a liability in your case).
Feel free to get in touch (email in my profile) if you want more guidance.