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?
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).
