Trapinch doesn't deserve any rare candies but it can be a fun trophy defender as you said. Yours is high enough in level that I personally wouldn't invest any more in it. You'll get all the use you can from it as it is, if you use it at all.
Trapinch are vermin in my biome and even in the massive numbers that it spawns, I have never used or seen anyone use a Flygon for anything other than gym fillers. Any of the other mons you listed are leagues ahead of Flygon and worth your time and resources.