It appears that you have stem girdling rocks. The top is dying back as a result, and I would expect to see more dieback in years to come. Frankly, I don't see stability as an issue. Oak is strong and rot resistant. The problem you will face is as you remove more and more dead, the tree will start to look unnatural and become a removal. This will of course be accelerated if a septic damages roots.
I can't imagine a way to remove the rock, and doing so may cause stability issues, so that's not an option, from what I see.
My advice is to plant a tree as close as possible, and as this tree slowly dies off and is slowly removed over the next several years, the new tree will fill in and be well established by the time you do the final removal of this tree.