Currently, to shiny hunt yungoos, the only way to do so it via overworld wait, which is extremely annoying as you cannot just honey and try to spawn it.
Yungoos, in USUM, is a early game (route 1 to 8) pokemon, with the gumshoos trial being in a cave. And honestly, there a lot of places yungoos line could fit.
First of all, a savannah is a good place for it. Mostly plains, some grass. Fits. Sticking to the 2nd theme of the trial of being in a cave, Gritty Burrow, Glitter Cavern, and Tanren Tunnel are all options that come to mind. I personally lean more towards Tanren Tunnel as it’s an early game dungeon that fits that early game pokemon theme, however, Gritty Burrow or Glitter Cavern may be options as their recruits are less.