I usually do those 35 deg slopes downward, but have done them up....and turned around mid-slope. Routinely do 25 to 30 deg sideways though....but can still side-slip bit more than I like at times (damp or very dry grass) ....the main parts (75% of which are quite steep with 20 to 30 Deg sections) I do closing-the-square (so blowing out), so get to deal with all angles from all directions. Experimented all ways on the very steepest section when I first got this machine and before any mods....satisfied that Could Do everything....but bit more peace-of-mind (and for the wife, who still does not like to watch me on 'Bracken-Fern Hill') with the 6" wider main-track, loaded tires, and more counter-weight. Several friends tried to talk me into sheep....and others still hardly believe I actually Mow the steep hill.
Do not tell anyone, but I also bypassed the seat safety-switch....going down the steep slope one's butt gets light on the seat and can (has) shut down the engine....and of course also the HST....not a good feeling as the brakes on my machine are mainly for parking, not smooth controlled desent from near-panic mode! I also engage the diff-lock and locked-4WD mode to maintain full control. I am quite safety consious, but keeping the engine/trans running is safer than shutting off!
These Kubota F-Series are great commercial-duty mowers....but mainly just mowers....(they do have a snow-blower and snow/leaf rotary brush available)....and as you have probably figured, can tackle much more severe terrain than any ZTR. The Ventrac machines are more multi-purpose with a range of factory attachments.... but still great for steep terrain... with dual wheels....both quite $$$$ new....would seriously factor in local service & support in your ultimate decision.
Back in Colorado I had an articulated PowerTrac 1845 for our very steep property there (see a pattern?)....and seriously considered shipping it down here, but few of the attachments I had for it would have been useful here....and I did NOT have a mower head....so sold it with that place and got the Kubota for here. That PowerTrac is sold as a commercial Slope Mower and rated to 40 Deg with dual wheels F&R. Mine only had the foamed-singles, but had 3" wider track front & rear. Only factory-direct with no local dealers or service, but is really only an engine (Duitz) and hydraulics... and lots of heavy steel plate....very few special parts.
Cheers, Rip