(Hey, my original joke was that I was going to pretend Pyra & Mythra were a humanoid Morpeko who Types they changed, though I couldn't think what Type Mythra would be (Fighting?) so went with the above instead. Honestly think the joke I went with is better)
I'd go with Steel. In addition to STAB Flash Cannon, all of the other light-element blades would be Steel-type regardless of what we classed light as.