A Hypericum I would say. Can't tell you which one without some idea of habit and size. Candidates might be H calycinum or H patulum. It might also be a seedling from a hybrid like 'Hidcote' in which case it may not have a name.

Nope, it's not the mescal bean (Texas mountain laurel, Calia secundiflora ). That is the plant I originally bought that my mystery plant sprouted up next to, and it's definitely not the same plant. My mystery plant has much larger leaves than mescalbean. Also, mescal bean has no thorns. Thanks for the input!

...but all the same, a genus known to shed its leaves among the earliest of all in the fall. One can easily tell what ash they're looking at if the bud scale scars are present, the green ash having a "top" line of more or less straightness, while the white ash has a big smiley face curve. There are other ID'ing techniques, but that's a good one. +oM

If you need the room for the orchard filberts then you will want to take the existing native ones out. Otherwise it is a matter of personal preference - if you might like the native species enough to keep it there. Try reading up on it and seeing if there is anything about it that might interest you. Like non-resistant kinds of orchard filberts local plantings of corkscrew filbert are currently being spoiled by eastern filbert blight. So you may soon see your specimen develop large sections of dead branches. Even to the point where the tree is mostly gone. It says here the two orchard cultivars you mentioned above are highly resistant, so maybe you are attuned to this problem. If not you will definitely want to continue choosing kinds that are. Hazelnut (Corylus avellana)-Eastern Filbert Blight https://pnwhandbooks.org/plantdisease/host-disease/hazelnut-corylus-avellana-eastern-filbert-blight