I am trying to understand your post. Your tree peony scions were grafted onto herbaceous stock that was already rooted, right? Al

Yes.

I want to help them make tree peony roots as quick as possible so these plants can get established quickly.

I would plant them in the ground about four inches of the tree peony stem in the soil. You cannot force roots to grow. Tree peonies growers need a lot of patience. Al

I'm noticing only some roots forming on my grafted tree peonies that are a year older, but nothing significant so I pushed some soil aside, applied juicy roots rooting gel, and covered it up with a mound of more soil to hopefully promote roots to form sooner.