WS2812 definition, gimings, protocol ...: https://cdn-shop.adafruit.com/datasheets/WS2812B.pdf
SPI on Raspberry: https://datasheets.raspberrypi.com/bcm2711/bcm2711-peripherals.pdf
Especially taking care of "2.3.1. SPI implementation details", where they say that there is a garanteed gap between two bytes. During that time the MOSI signal is also low (alas it's SPI).