LG, thanks for starting this post. I'll be interested to see the progress of your plants. I wish you and Karen all the best with your adventures, with growing tree peonies in pots.
I'll have to admit, the tree peonies that l bought from one of our local nurseries remained in their same pot that I bought them in. I did supplement by topping them off with Staygreen potting soil, and they were fertilized the same way that my peonies planted that are planted in the ground.
My older tree peonies that I recently dug up are in large pots or the larger garden bags. They were potted with Staygreen potting soil that contains fertilizer, with additional Perlite added, to prevent compaction.
I mentioned in another post that my TP Baron TB had gone dormant, and after it had been potted, it put out new leaves.
I'm in zone 7a/b, (I feel that we're more a than b due to our microclimate). I've never worried about overwintering the peonies in pots outside, I've only ever lost 1 plant.
I've done both new roots, and well as potted plants in the past. I'll have to visit my SIL to see how the newly potted plants are doing.