I actually went with a design like you proposed in the first iteration. It turned out to be more time consuming than the SD card solution, specifically for my non technical SO. Now, she just stuffs an SD card into her laptop, transfers files, puts the card into a prepared cartridge, and sticks a label on it. The cartridges can be ordered fully assembled, so she doesn't have to solder anything.

With a fully mechanical solution, she would still have to extract the SD card from the player (or I spend considerable more effort on the software side, so the device can somehow also act as a mass storage device when connected via USB, givng access to the internal SD card), print or construct the "key", stuff the key into a cartridge and label the cartridge.

There's no great practical difference. The only difference is a higher per cartridge cost. Since that's around €2.50 and could be further reduced by bulk orders, I was fine with this design decision.