(Sorry if this has been asked before, I forget.) Is there any difference besides a categorical one between legendary and mythical pokemon?

Yes, actually. Legendary pokémon are well documented in a region’s history, and generally speaking, there are more sightings of them than mythicals. Mythical pokémon, meanwhile, are often only vaguely mentioned in a region’s folklore, and sightings of them are so rare there can be debate about their existence. The line between these two categories is fuzzy, though, and many researchers consider the latter to be a subset of the former.