Locking at the photos of your property, I would think a grapple would be your first priority. You already have a Bush hog and a zero torn to do the mowing, but could use another weapon for clearing out brush and debris from the unimproved areas.
I have a Bush hog, 2 flail mowers, and a Land Pride 60 inch rear discharge finish mower for turf maintenance on my property, and I use them all in different parts of the farm. The one I use the most is the finish mower . After initial cutting in the spring, the finish mower gives a cut almost as good as the lawnmower, without the clumping of the BH.
Lastly, the tractor seat is a lot better ride than the zero turn in the fields.