This project is now changed from Teensy 2.0++ to Raspberry Pi 3B. Why? The Teensy works great, and has much more I/O then a Raspberry Pi but the DFRobot parts: the DFPlayer Pro is particularly a garbage product that does not reliably play low-bitrate files without random stuttering.

So, the solution is to use an RPi. It has barely enough I/O to handle the buttons (24 pins total). This leaves no room for expansion.

See pin diagram in the comments. There are: 18 momentary buttons 4 latched buttons 2 LEDs