Gracious! My first thought in seeing the pile of parts was you could use a stronger machine!
But what lasts 600 hours? A spindle? The mower itself? At 25 hours a year isn't that 24 years between failures? At that rate it sounds like you don't need a heaver duty machine.
I have a somewhat similar situation in that I use a mower outside it's envelop of expectation. I have two Kubota 60" zero turns with 25hp diesel. I ran the first one hard with both mowing and for "bush hogging" in the woods, heavy brush, etc. Often hit stumps, branches, rocks. Mowing in the rough gets noisy sometimes but nothing has ever broken on the machine, I guess it's built strong. The biggest problem was slowly losing air in the front swiveling tires and replaced the serpentine belt once in 15 years.
I finally bought a new one and decided to keep the old one for the rough and save the new one for the grass.
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But I never heard of using a mower on horse manure. I made a drag from a 12' length of chain link fencing and pull it around the pastures with a 4-wheeler.
JKJ