Is $6,000 to pour concrete 2ft deep for deck foundation a fair price?

I am undertaking my first big DIY project, building a deck around a stock tank pool, using Maker Grey plans with modifications. We initially put down gravel and leveling sand, but after rain, the sand settled about 2 inches, leading to concerns about soil erosion and long-term safety, especially with North NJ winters. I was hoping to lift, move, and re-level the deck, but a contractor quoted $6,000 for a concrete foundation, which I cannot afford. I am now considering using the existing frame for a ground-level deck in a more level area, potentially with a short retaining wall, or rebuilding the deck with adjustable pedestals. I am also looking into quotes for a gravel bed, but access issues (27-inch wide alleyway, no driveway) might make it expensive. I don't want to spend more than $2,000-$3,000 more on this project.