Glad you shared this. I ran into something similar when I tried using a Pico W as a USB HID device, hardware was up to the task, but the firmware just wouldn’t let me get where I wanted..

Using Arduino and serial is a smart way to go here. Still, it’s worth remembering: the Pico W has built-in WiFi, so you don’t have to lean on serial if you don’t want to. You could have your temp-reading app send control signals over HTTP instead. No need to tie up the USB port, and the connection isn’t killed if someone yanks the cable.

And yes, big agreement on the watchdog or failsafe. Any fan controller that doesn’t ramp up to 100% if it loses connection is just waiting for trouble. Fire risk is real.