Trim back to trunk. Your tree appears to be a grafted one and they are coming from your root stock. If you like the shape of the leaves etc you can try rooting one of the cut pieces.

The picture is not great, but, from what I can see, it does not appear to me that the sprouts are coming from a rootstock. It looks like they originate along the entire length of the trunk. This could indicate some type of damage to the trunk. Does the upper canopy appear vigorous and healthy? If not, the tree may be suffering from some type of vascular disease.

Yes, the sprouts are along the entire length of the trunk.

Upon closer inspection, the upper portion of the canopy that has grown in the 2 years since I planted the tree is doing great!

However, in the lower portion of the canopy that was present when I planted the tree, there are a lot of small dead branches. Is that normal?