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This looks great. It's the smallest ESP32 board I've ever seen.

There needs to be an ecosystem of "hats" such as OLED screens and environment sensors and buttons, so that it's easier to build simple projects.

He has them!

The Lilygo TTGO Micro32 module with the ESP32-PICO-D4 chip:

is smaller - just 19x13mm! Though it doesn't include USB connector, power supply, USB-serial IC, etc.

It's used in the Open Smartwatch project (https://open-smartwatch.github.io/).

That's a module - it cant be used for anything unless you mount it to a bigger carrier PCB and add all of the extra components.

This is a better comparison - full dev board, just add your own USB connector of choice . https://www.tinypico.com/tinypico-nano