Simple way to drive a lot of neopixels fast via USB
The protocol is as dumb as it gets: send length of the data to be displayed on two bytes, than send the data for each pixel
The default runner is picotool and you can get it via brew install picotool, or compile it yourself. The benefit of using it is that thanks to usbd-picotool-reset it is able to reboot the pico into BOOTSEL mode, so you don't have to worry about it.
Alternatively, you can use elf2uf2-rs by switching to that runner in .cargo/config.toml, after installing it via
cargo install elf2uf2-rs --locked