There exist analog keyswitches, have you considered playing with those?

I'm really interested in this idea for other DIY instrument purposes.

Razer has keyboards with optical ones, and some people have made them by putting inductive sensors underneath mechanical switches, but I couldn't find any analog keyswitches for sale. Do you have some examples?

How can you deal with a large number of analog inputs to a Raspberry Pi or similar?

I don't remember how many analog input pins exist but a separate circuit that digitizes many analog inputs to a serial stream would be how I'd do it, personally.

While it would be interesting to play with analog keyswitches, it would depart from the experience of a real sax, where the "half pressed" key is of not much use. And it would dramatically increase the complexity of the design.

But the full design is published, just in case anyone wants to try :)