I haven't figured out the complete picture of what I want for the controller, but it's almost a certainty that I'll use an ESP32. As I write this, the pin assignments are a bit arbitrary and only intended to convince me that I have enough of whatever pins I need. I'll be updating this table over time.

Although I expect to only use 3 fans, I have enough GPIO pins to design for 4 fans (and even a few left over). I expect to use a PWM pin and a tachometer pin separately for each fan (F1, F2, F3, F4). Most PWM fans don't stop completely even at a 0% duty cycle, so I'll use a MOSFET to explicitly control the power lines for each fan. I'll use the I2C interface for a temperature sensor. I'm not sure yet what button, rotary encoders, LEDs, or display I'll use.

This table is based on the more or less standard ESP-WROOM-32 development boards with 30 or 38 pin headers. I plan to use the 38 pin variant. The I/O recommendations in the Specialization column are based on the chart from here (I've seen similar charts on other web sites; this is the one I consult). "I" means OK for input; "(I)" means OK for input with some caveat. Similar notation for outputs.