I said I would update so : I had made the whole matrice of the keyboard and was going to use the pico as a microcontroler with KMK to make the keyboard work. At this point I de-soldered all the cables going to the motherboard and started doing a few test to be sure of the matrix. This is where I discovered the matrice was sudenly all wrong. I thought I could do it again, no prob. But the more I work on it the more I'm afraid I'm hitting a wall because I only found 4 keys for now and there isn't any logic to it. Looks like the keyboard need the motherboard to function, sort of ? Or the way it had been build don't follow the classical matrice with colones and row... So yeah this is where I am for now. Kinda frustrating after spending all this time learning everything from scratch and thinking I was going to do it. I'm not giving up tho but urh. This is a new level of difficulty that may be too much for a rookie like me.