This is great. Is there some tutorial of this caliber for Pico's PIOs?

Check official documentation of the RP2040 and see if you like that. It's chapter 3.

https://www.raspberrypi.com/documentation/microcontrollers/

Some interesting excerpts:

There are two PIO blocks with four state machines each.

Each state machine, along with its supporting hardware, occupies approximately the same silicon area as a standard serial interface block, such as an SPI or I2C controller.

Making state machines programmable in a software-like manner, rather than a fully configurable logic fabric like a CPLD, allows more hardware interfaces to be offered in the same cost and power envelope. This also presents a more familiar programming model, and simpler tool flow, to those who wish to exploit PIO’s full flexibility by programming it directly, rather than using a premade interface from the PIO library.

The Pico’s official documentation seems leagues better than what the PRU’s offer(ed).