And even a 7$/ku MCU is complete overkill (assuming that's the real price, not shortage-related, though its lead time is over a year, so the price is probably

And even a 7$/ku MCU is complete overkill (assuming that's the real price, not shortage-related, though its lead time is over a year, so the price is probably inflated), because literally any microcontroller with an ADC can easily do this. Around ten years ago I built my own on a tiny AVR, and even that would be an expensive option compared to what you could use instead in a commercial product (<0.5 $/ku ARM MCUs). I haven't had a fan controller for many years though, because they're quite pointless if you do things right.