You could go with a an x749 with 3ph. Pretty fast mower. Or just about anything bigger. Bang for your $ would be LS, Mahindra, Kioti, bobcat. Just about all of the SCUTS and small CUTS would work for you. If you are looking to save time, keep the x500 for the "lawn" portions and get 35+ HP with a 6' bush hog for the fields. With a bushhog you could get by with cutting once or twice a season.
I only have 2.5 acres, but only cut one of the acres about every 2 to 4 mowings. This saves time without it getting too out of control. If I cut the 2 acres or so of grass, it takes me about 1.5 hours on my JD425 AWS. AWS saves me bunches of time and no weed eating.
A real good choice would be the Mahindra Max28XL