This is really cool, I'm doing something similar! I'm making synthesizers with the Uno and Due; I haven't tried doing anything with audio input yet, but want to play with that as well. How is the sound quality of it? I did the 8-bit resistor ladder DAC from the instructable linked, but quickly broke the layout when I tried to bring it to a friend's house and haven't tried putting the breadboard back together. I really should go back to that, I liked how crappy it sounded. I also tried making a 1-bit DAC out of it, which worked better than I expected (though it still wasn't good, I just didn't expect anything haha).
Now I'm using the Due and Teensy (Cortex M3 / M4) because of the hardware DAC and 32 bit processor. I haven't gotten filters working yet either, they are my next goal. IIR first, and then FIR if the Due can handle it. But I have gotten a cool resonant filter simulation by using a hard-sync as described here [0]. You get some neat variations by changing the waveshapes used and the windowing function. I might not even have subtractive filters in my final design, just because of how cool it sounds to approach it differently, but I still want to understand how to implement them.
Nothing on Github to share at the moment, unfortunately. I'm too embarassed by the state of the code, lol. But I will be checking out your project this weekend, so thanks for sharing!
[0] https://en.wikipedia.org/wiki/Phase_distortion_synthesis#Sim...