On the contrary, sapper is the best pve class in the archer tree. Cos might be referring to kcbt1, where sapper had long cooldown on all their skills. They’ve all since been reduced.
Anyway your sapper build is looking real good. IMO QS rank two isn’t really worth it. It’s looking to be one of the weaker classes in the game. Paviase doesn’t provide great cover, caltrops don’t work on flying enemies and stone shot is more for knockback and stun than damage. I’d say get QS just for one circle, get sapper earlier and take something else later down the line. Scout would be a good choice for cloaking and camouflage, but it’s all down to personal preference and how many circles you want to take each class to. (For a total of 8 ranks iirc)