Alex, the largest hole that I ever dug for peonies is probably 16" deep and 16" wide but with many others in between. Then I added 4"-6" to the top of the soil so that gives me about 20" deep hole into which I mixed in composted mushroom substrate, shredded leaves, peat moss, lime, and some bulb fertilizer with the soil that I dug out. Some parts of my yard have good soil, others not so much so i modify as I go. 80% of my peonies are on raised beds of 4-6" higher than above soil level. I don't think I have the best soil or prep for the peonies but I guess just good enough to give me some blooms every year. So vigorous plants do very well and weak plants not too well. In the case of an underperforming plant, i try to find a location that best suits them. Of all the peonies that I have, I think the most finicky one is Salmon Dream. It does not like root competition nor bad air circulation. It is currently not very happy. I had to remove daffodils and perennials around this peony in the hope that it will do better next year. Some of the easier peonies to grow are Coral Sunset, Red Charm, Nova, Mackinac Grand, a mislabel White Cap. These are not fazed at all by bulbs, perennials and even shade for Coral Sunset and Red Charm.
