Really exciting to hear about your plans for introducing Piezography to your shop! It's definitely a niche market, but the quality is hard to beat and there are certainly customers out there willing to pay a premium for that.
I think your approach of aiming at photographers selling their work online and at shows is smart. They'd be more likely to appreciate the quality difference and could justify the cost if it helps their work stand out.
As for the P6000 vs P9000, it might be worth starting with the P6000 to see how the demand is and then scaling up if necessary. The initial investment is less and it might be easier to move the volume of prints needed to make it worthwhile.
Best of luck with your venture, it's an exciting field to be in!