To preface this, I am an Arctic Spa dealer, not your dealer, but I do sell Arctic Spas. One advantage that you have purchased is the Arctic Spa ForeverFloor. This is a full structural floor not just a waterproof base like you will see on the nicer spas out there. You can set an Arctic Spa on nothing more than level solid ground. What you have is fine. You have actually done more work than what is needed. This will give you the drainage, etc. As long as it is even, the ground will move slightly, however, since the hot tub is not connected to anything, this really will not be noticeable.
Let me know if you have any other questions.
Happy Hot Tubbin',
Christian Staples
Arctic Spas Utah