On our private lake we are setting ours fairly close to shore so that it can be reached by an excavator. And driving the posts down with the bucket off that. I don't know if you're accessible to something like that (My dad owns an excavating business) But something to think about. Probably the easiest way.
We installed ours with legs that are metal, have 3 foot long square tub feet that reach out on each side of the legs and then installed 60lbs weights on each end of the feet. It is rock solid. Hook a chain and pull it out with a pickup.
I would use metal as well, then at the 3rd/4th foot from the bottom have a metal plate welded so it will have down in the dirt 4ft then maybe a 2x2 foot each leg will stand on.
I barely understand what I wrote so it would look sorta like this