This is why BeagleBone uses the Ti Sitara SoC. Besides that arm core(s), it has PRUs which are custom made for precisely this sort of thing: time sensitive bit banging.
basically, this still looks hard. also, i wonder how other applications suffer after all the hoops the OP jumped through, especially networking. looks like i'm going to continue adding a little real-time microcontroller (like a blue pill) just for driving ws2812 to my projects, thankfully the ws2812 tend to be so much cheaper then the spi versions that the cost still ends up being less
BeagleBones are awesome for these kinds of things. They are very similar to RPis but in addition to the ARM core they have two embedded hard real-time capable CPUs (dubbed PRUs) that run at 200 MHz.
If you are on a budget the Arduino is perfect to drive those because you are bare metal and there is no interrupts...