If you haven't already, go back to the previous step at https://pianoclack.com/forum/d/637-build-your-own-hybrid-piano-step-four-the-cables (and to all the previous ones linked in a daisy chain).
Ok, this is pretty lame post, only to show I've done something and to have something out there in the forum in case people are bored. The tl;dr; version is: I'm simply following @JayKominek 's footsteps.
In more details, I saw an in-stock Nucleo board at DigiKey with the exact microcontroller I needed, so I jumped and bought it for experimentation. The website said they had only one in stock, so at first I thought of somebody returning theirs way past the typical 30-day and DigiKey accepting it because of the high demand. Oddly, after I bought that, there was another one in stock, and it remained in stock for about a week. At the same time, these boards popped out everywhere, including the manufacturer's (ST) own website, where it even had discounted price and free shipping!!! I thought the shortage was over!
I was tempted to buy another one (or more) but I did not act immediately… bad move: the boards disappeared from all sites the next day. If I had another one (or more), I could have played main vs ADC board(s), and that would have been very fun (and close to the final arrangement in the piano).
Anyway, I played a bit with the various demos of the board, and everything worked fine. A little more confusing that I'd want, but fine. I loaded @JayKominek firmware for the ADC board, and that also appears to work okay, however I've not fully been able to see the hammer movement in the firmware's variables' values yet (I did see it from a demo ADC with RMA, so I know the hardware is working correctly).
Now, the mandatory pictures to prove I've done it
Not sure anybody is following the project that closely, but somebody may notice that I am using a voltage divider (which in turn will modify the phototransistor's bias level) rather than the transimpedence amplifier. I was eager to do stuff, so I jumped straight there, I have the parts for the latter and plan to do that too, perhaps on a different channel to observe differences in behavior.