@petewill Ah okay, understandable. I thought about the pros and cons of using two MPR121, since they are extremely cheap (1-2$?), but it would mean more cables, so it would only be worth it if it improves accuracy. I finished rebuilding your setup a few hours ago but didn't have time yet to test it much, maybe I'll build the "virtual 13th pin"-variant aswell if I find the time.

@petewill
Did you try to use the "virtual 13th pin" of the MPR121? It can be used to have the MPR121 automatically combine the values of (for example) the first four pins and was specifically designed to handle proximity detection instead of touch.
I wonder if this could theoretically cut down on the necessary calculations.