Puck.js and Neopixels – Possible?

I've got a Neopixel Ring (12 LEDs), and two Pucks. I know they run on different voltages, so I've connected the Neopixel to a 5v USB supply, and used an opto-isolator to drive the data pin at 5v from the Puck.

I can't figure out how to make it work, though.

I've cobbled this together from various posts that I've found, but I assume I'm doing something wrong (or the Puck can't do the data rate I need perhaps?):

const PIN = D30;
const LEDS = 3;

function main() {
  pinMode(PIN, 'output');
  // I'm using 400000 here because I read they need a 4- or 8kHz stream, but no dice.
  SPI1.setup({ baud: 400000, mosi: PIN });

  let arr = new Uint8ClampedArray(3 * LEDS);
  let idx = 0;
  for (let i = 0; i < LEDS; ++i) {
    arr[idx++] = 255;
    arr[idx++] = 0;
    arr[idx++] = 0;
  }

  console.log(arr);
  SPI1.send4bit(arr, 0b0001, 0b0011);
}

main();

I've confirmed that the isolator can do upwards of 50kHz, and that it's working (by using a plain LED and setting D30 high – it lights from the 5v rail).

Help! Thanks in advance.