A couple of handfuls of bonemeal in the bottom will help as it breaks down over time. Replace the mixed soil into the hole until a slight dome forms above natural soil level. Soil settles with time, so the dome will end up level with the soil in a year or two. I always mulch the first year to prevent the heaving and exposure of these large fleshy roots. In early spring, gently and carefully pull the mulch back from the new growth.
Peonies are light feeders.
Too much fertilizer in any form, especially ones high in nitrogen, produce negative effects and can injure or kill. Some compost and bonemeal scratched into the soil around the new stems each spring is sufficient. I find Peonies available at local nurseries and garden centers in Bomb, Rose and occasionally Japanese flower forms in limited selections.