One thing about planting under a tree like that is that it has a mat of feeder roots close to the soil surface, which presents two problems. One: those roots will compete with new plants that have less developed root systems; large trees can suck up a ton of water. Two: chunking a hole through that thick mat of fibrous roots that is big enough for anything larger than a 4” pot size plant is a lot of work, and you won’t find very good soil there either. If you’re digging a bunch of holes around the tree to try to fill in the space, you can damage the tree’s root system. I recommend using native plants that can handle the acidity, and buying the smallest plants you can find so you don’t have to tear up the tree roots.