Nope, not programming an µC yet, I started with the simpler APA102 protocol, and here is the result :

I currently test with 24.5 bytes per A4 sheet (4 start bytes, 5x4 byte for 5 leds, 4 bits stop)

Here is the strip I used :

Black means 1, white means 0, this way you read 0s when no strip is inserted. The signal is fed into the negative entry of the opamp, then the opamp will output 1 when light is blocked

As you may have noticed, the G5 (global luminosity for led 5 which also is a start byt for the led5 frame) is wrong and should start with 3 "1"s instead of 3 "0"s. Once this has been corrected, everything worked fine :)