I know you don’t want to kill the grass long term....
the original, cheapest-available Round-up weed and grass killer will kill what is there without preventing new stuff from growing there later — actually even the long acting kind will work the same way, but you don’t need to spend the extra money...
Glyphosate, the active ingredient in Round-up, kills by disrupting the enzymes in plant cells preventing them from being able to move water through the cells. Furthermore, it becomes completely inert (inactive) upon contact with the soil. It won’t hurt people or pets (well, unless you actually drink the stuff— but their website says it is not toxic to people or pets when applied as directed, and the directions say to exclude pets until it has dried.... not a problem for any future chickens). Round-up will not prevent new plants from growing in the treated area.... so you can sow new grass seed there later, of a type you really want.
The problem with going a chemical route in cases like yours, is that even after you kill the grass, you’ve still got to remove it..... grass that high can’t just be tilled under once it’s dead. So assuming your goal isn’t 6 foot tall dead grass/weeds, you’re still going to have to cut it down.
One of the power tools people have already mentioned would probably drive any wildlife out of the area as you use it, but I know I would still be apprehensive moving through the brush, and especially picking it up afterward.
Personally, with the possibility of venomous snakes in the area, I’d look to hire someone— human or goat— to do the job for you. Honestly, you’d have to do some calling around, but I’m not sure goats would be much cheaper, because if you are renting them say, by the week, it’s gonna take a few weeks for them to do their thing, and they won’t clear them out completely, the roots will still be there. On the other hand, if you hire someone, either a single person or a crew, it would be finished in at most a day and likely just a couple of hours. And you could specify whether they remove the roots and all. Of course, once they’re cut down, you can spray the roundup then (assuming it’s not too hot where you are— it has to be below like 90 deg. or so to apply it and have it work properly, otherwise you’re just wasting money.), and till it under in a few days.
So I guess it comes down to how fast you want it done.