As I was putting together the breadboard for the receiver module, I made a horrible realization: I don't have enough IO pins. I forgot about the CE lines for the SPI chips. I'm sure I could do some trickery to make this work, but in the interest of time, I'm switching both MCUs to be ATMega328s.

This can be revisited later down the track. Maybe an ATtiny84 would work - with 14 pins, available in DIP, and half as much per piece at a 328 ($1.41 vs $3.23).