Just want to say- this is a really neat idea and project!

p.s. thanks for the additional sourcing details

AliExpress and Adafruit, e.g. https://www.adafruit.com/product/5867

Have you thought about ways to make the lights detachable so you can wash the garment? Or is the trade off at a point where redoing it is easier overall?

It’s maybe plausible to gently wash it with the filaments still attached? I did actually use connectors so the shirt could be detached from the drive electronics, though I’d be a bit worried whether the stitches would survive and corrosion on the solder joints. Adhesive/waterproof heat shrink could maybe help?

The other option is maybe attach the filaments to an embroidery style patch instead of the shirt itself and then attach it with Velcro to the shirt?

Magnets?

You could use shrink tube around the solder connections. That would limit water ingress though it's not perfect. It's what I always do, it also gives more mechanical strength so you don't break off the the wire easily.

Speculating here. The lights and the electronics could be seen onto a fabric envelope, and the envelope could be attached to the final garment through hoops and loops (Velcro).

I wonder if white thread would've been more invisible, the black thread seems more obvious against the LEDs? Being nitpicky, its a cool project for a single night.

There's actually a specific transparent thread you can use for this, it's called a monofilament polyester (aka "MonoPoly") thread, in clear or "invisible" variant. Would be perfect for this project.

I was gonna say maybe fishing line would work, but I think maybe some fishing line is some sort of monofilament polyester?

Or color matched. Hadn’t even thought of that to be honest

this is a really cool project! In the future you might want to print the paper designs in reverse and put it on the inside. Any paper remnants wouldn't be visible. I think that's how a lot of embroidered products do it, and I've had some hats that leave a lot of the material instead of tearing it out.

curious, why use black thread for red light? Haha..

Cool project. The best part is the actual pictures, you can tell taken with care by a human and the project is well described. So hard to find decent writing these days.

I think for your next big idea create a tee that can match any company's swag brand; I am just tired of managing tees that fill up my drawer, I wish I had one that could be me all. ;)