I'm didn't read this whole thread yet so I'm not sure this has been pointed out. That is any compact,sub-compact, or heavy duty L&G tractor PTO driven auger is NOT reversible. IE, if you are drilling deep in very rocky soil there is very good a possibly it could get stuck. I live in upstate NY where the glaciers have left us many, many rocks of all shapes and sizes(and sometimes mixed with clay). I have built a half dozen decks and rented the towable, hydro driven , (and reversible) auger each time. If I did not have reverse, I would have been up a creek without a paddle several times. The other issue with PTO driven, 3 point hitch ones is they have lift but not down pressure so you have a manual lever and your muscles for that. Again, from experience, down pressure would be a big help in compacted clay.
I assume the mini skid attachments are hydro driven (and have reverse) like full size skid steers. That sounds like your best bet as others have said for what you need. I know they are very pricy (over $20K ??)and around here rent for about $200 a day. I guess I would rent one first to make sure it works like you expect and gets the job done and most important helps you make money. Just my two cents.