It’s an older mature orchard with some space.
I’m mowing with a 6 foot finish mower currently
So you are currently using a 72" mower, but one issue (from my experience) is that it is inflexible mowing while turning, whereas a ZT is designed with perfect flexibility.
I am outside of Houston and understand your environment. We have a diesel JD 997 with 72" deck. My wife is the operator and for the 6.5 acres we mow as yard, it takes her the better part of a day, every 10. Part of our issue is the lawn is not as smooth as your area, so she slows down, she picks up all sticks and twigs into piles so I can remove them, also mows driveway ditches and around the pecans along it. The JD is a beast. We bought it used, have owned it for 10 years and added 1200 hours. Benefits: extremely reliable, very tough engine that endures overheating when the mowing debris clogs radiator flow, refuel every 20 hours or so of use, no perceptable wear after all these years, deck is super tough and air/grass flow is extremely good, chops up twigs and such up to 3/4" with no concern, but I have been known to do larger. I keep telling my wife to "cut them up" - next time you won't even see them. Deck cut range is 1/2"-5".
At the current time, the only thing I wish is that the mowing width were greater to reduce time because of the acreage. I have considered doing a special design by adding a couple of 21" gas-powered hand mowers, one on each side of the rear tires. If you look at the posting by 'chim', he is using a 90" deck - great idea. There maybe multiple PTO-based mower decks with a wider path that use a hitch instead of the 3 point arms. Then you can just add a loader to your current tractor for other tasks.
If you would be happy with a taller cut, I might even suggest a batwing mower with sharp blades. My 15 footer (180") can drop down to less than 4" and if it were dedicated with blades kept sharp, you would be done in a snap and still be able to circle your trees with it.