Concept: if Mirio can permeate through stuff, could he do the same with light particles, effectively making him invisible for as long as he holds his breath? Like Luz's Invisibility Glyph in Owl House?

I doubt it. I could get into the science of it, like how "Permeation" means physical matter passing through physical matter which is waves and light works as both waves and a particle thus meaning that it can still interact with him to some degree hence why we see him. The ultimate point is that light still affects Mirio and I can't see it him being able to train "Permeation" to avoid it. The Quirk is either on or off. Mirio's growth with it comes not from changing how the Quirk works, but how to work around and with it to get the best results.