Very nice to see new 'clackers and DIY'ers! Wondering why you went "your way" with Teensy, rather than embracing the very nice stuff that @JayKominek put together (hope that does not sound like a rude question, it's simply a sincere curiosity).
T-Ro zero coding experience and zero pcb design experience
You may (have already) read some of my rants on PCB design on this forum. I hope to get used to it soon.
If I understand correctly, you are taking the string off this instrument? If so, you may take the felt off the dampers and that should simplify the placement/response of the sensors.
Alternatively, you could put the damper sensors on the keys. Since the dampers and the keys are directly connected without escapement, this is mechanically equivalent. However, the best place to put the sensors would be the back of the keys which poses two issues: I am not sure if there is room in your instrument, and you will need to remove the action from the piano. Usually the latter is considered a big deal on an upright, but it's really simple, if a bit time consuming -- if anything the problem is that once removed the action will be at extreme risk to be damaged, since however you place it on a surface, it will be putting pressure where it shouldn't. If you go this route, consider building something like
Alternatively, put the sensors in front, but again, space may not allow you to have an easy life with it. Be very careful with the keyboard lid, which sometimes needs more clearance than it is obvious. And also, stay as distant as possible from the pin, where the key is not moving much.
Best of luck!