definitely not ideal and some significant work but could you use a tow behind drag harrow / driveway grader??. They are cheap to buy and would this get the job done??.
I have a logging winch so I would make it a 2 person job. say maybe 50' of chain on both ends of a drag would let you cover a 50' length of hill at a time.
- set up a drag harrow at bottom of hill with a small atv or similar
- tractor with / without winch on top of the hill
- with tractor pull or winch the drag / grader cutter up the slope breaking up the cactus
- pull back down hill other direction with atv
- set up drag width of cut over and repeat.
- once all cactus cut free let them rot or rake to bottom of hill and pick up with grapple??
I have never worked with cactus so I have no clue but using 2 machines one at base and one a top "tick tocking" the grader - drag up and down the hill with 1 operator and tractor pulling up and the other operator and atv pulling down might get it done.
100% less than ideal but with working on steep slopes a possible solution??
this way neither piece of rubber wheeled machine on the slope or where cactus is only pulling the drag / cutter and and down the slope using the cable / chain hooked between the two machines. This pic has solid draw bar so I would unhook and use chain on both ends so I could cover large runs without resetting.