It's likely that your local authority will tell you what needs to be done, either directly or through the local septic installer.
Mound systems are an alternative to aerobics, and you may be able to continue using your existing tank. The mound can be located uphill, farther from the lake than the leach field.
Septic tank inspections that would let a buyer actually know the system's condition are very difficult to do properly.
Good luck!