If you want to make your own SD card
There are "simplified" specifications available on the sdcard site.
The first simplified specification is called Part 1: Physical Layer and purportedly includes the full command protocol (minus PCIe and NVMe extensions)
The storage cap with SDXC is 2TB, and with SDUC is 128TB.
More docs at https://www.sdcard.org/downloads/pls/ .
since something is clearly holding the mainstream market back all these years