Cruise southern fiords
The further south the fiord, the fewer people. If you want a cruise in one of the most remote places on earth, then you need to travel further south than Milford or Doubtful Sound, and join a cruise taking you somewhere really isolated, such as Preservation Inlet.
These cruises are longer (often seven or even ten days), and often involve fly-in or fly-out options by helicopter, and therefore are always inevitably more expensive. But after a week or so in paradise we doubt you'll regret the cost.