This looks interesting. Have you considered down the road giving developers a way to order labels, cases, and inserts to distribute games?
Yeah, I'm checking out the options now. I'm imagining something the size of a CD case, but with a foam cut-out for the SD card inside. That should be simple enough to get done cheap. Printed SD card labels are already a cheap item you can buy from AliExpress.
The cards themselves could just be off-the-shelf SD cards at first - it's a standard feature of an SD card, that you can switch it permanently read-only. (Making a card partially-read-only is a huge pain in the ass and companies charge a $30-$40 premium per card for it. That's why the Neki32 console has separate savegame memory instead.)
Fancy copy-protected cards are going to take some work. I'm confident that I can write the firmware for it, but getting it physically into a card could be difficult. The first model might look like a Turbo EverDrive, all piled on a circuit board. (I did consider making a separate cartridge form-factor, to make this part easier. But then I'd need to develop the physical cases for the carts, as well as the slot and connector on the console, and would lose the economy-of-scale of SD cards.)