Not sure I would call Goomy's evolution a "decent pace" since it normally takes a long time for a Dragon type, let alone one from one of the pseudo legendary lines, to level up and eventually evolve. Plus, this is the 2nd or 3rd time we've actually seen it battle, if you count the two times it fought against Team Rocket. If they wanted to flesh out Goomy's evolutions more, I'd say the writers could've more than evolved it into Sliggoo before the Laverre gym & then wait to have it evolve into Goodra until just before Ash faces the Snowbelle gym
Everything evolves at Goomy's pace.
Ash boxes the spares for new captures.
Gotta catch em all.