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.
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?
Ooo this is cool, I didn't know these exist. I once half made something like this with EL wire but I stopped after getting an electric shock from some EL tape. Turns out the edges are not at all insulated and you need high voltages to drive them. I'm not wearing that. 24V sounds much nicer.
p.s. thanks for the additional sourcing details
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?