It's not surprising that you are breaking plows. With tracks on your 6x6 you have a serious amount of traction... more than a plow designed for an ATV can handle. With the amount of snow you get, you are no doubt giving it a serious workout. With the amount of force you are capable of exerting, you are probably into the realm of a plow designed for a subcompact tractor (or possibly even the smaller end of the compact tractor range).The problem with continually upgrading the construction of the plow is that you will eventually discover what the next weakest link in the system is, and that next link could be something a lot mor eexpensive and hard to fix than an ATV plow frame.
I run a 66" plow on my Honda Pioneer 520 with diamond pattern chains on the rear wheels only. People debate whether to go 60 or 66" on the P520. I went with 66", since I have spacer on my wheels, bringing the overall width to 53". A 60" plow would just barely cover the wheels when angled. I wanted a little more width per pass. For a really heavy snow, I use the snowblower on the 3 pt hitch of the tractor and/or the bucket on the FEL.
The UTV is much faster, when the snow is light enough to handle it (deepest I've tried was only about 10" of "medium weight" snow last year, but I had no trouble with that.)