I mow up and down a fairly steep hill couple times per year. As I mow it, I get more comfortable but am still very cautious. Near one end, it gets too steep (tractor won't back up hill, slides down, even in 4WD). I worry that with the steeper hills, my engine or HST could starve for oil. I read about the HST pump having a problem on my tractor model and there was a upgrade to fix it, I rushed out to look at my tractor, thinking the whole time I had ruined something.. but the upgrade was already done.. whatever you find for solution, remember to not just check the slope you are comfortable with, but make sure the equipment can run reliably on that slope..
Good luck on your journey!