So let's get to the juicy bits of this project!
To be quite honest, the hardware couldn't be any simpler. A few 3D printed pieces can be glued together to form a seven segment display unit. Each individual piece contains two WS2812 LEDs, cut from one of those cheap LED strips you can get on eBay.

The LEDs are chained up like you would with any 'ol WS2812 strip, starting at the left segment and going in a clockwise pattern, before passing through the center piece and into the next segment.

The brain of the whole thing is a single ESP32 Dev Board, which you can get off of eBay or similar for just under 10€. They're really beefy things and, once you have the ESP IDF set up, really fun to use, too!
They also include a "IR" sending/receiving hardware unit, which can send WS2812 data signals with almost no CPU time needed - now THAT's handy!

The first proof of concept can be seen above. I've used a few of my self-written libraries here to mix colors, add an overlaid "brightness control layer", and also add a delay between each pixel updating, which adds up to this really cool pulsating effect. It's hard to see on camera, but expect some higher resolution shots once it runs more consistently, and once I add a few more digits!