This has been going on for six (6) years. How
do I get this process moving faster?
Unfortunately, there's no way to get this process moving faster unless you just want to pony up the full cost of decontaminating your property.
Do I have a solid suit for damages?
No. Not solid. Nebulous at best.
Unless there is strict liability by statute, you would have an almost impossible time proving that anybody was negligent with regard to the tank leaking. And it takes negligence for somebody to be responsible for somebody else's damages.