Need Suggestions for Pistachio Butter

Well you cannot expect to label the new product as pistachio butter. And previous customers who purchased the pistachio butter will not be happy about the new product which is a pistachio flavored spread.
I think you could do pistachio butter at 30% pistachios and also create a new pistachio spread product that is a lower price for customers with the 15% pistachio and use more dairy products to call it a pistachio cream spread- avoid adding more hydrogenated vegetable oils which are unnecessary

Also, I'm not an expert but is there a legal minimum percentage of pistachios thst it has to contain to be considered pistachio butter and not flavored spread?

To be honest, I can't see a reason why you would want to reduce the amount of pistachios in the butter. At 15%, what you are selling amounts to pistachio flavored fat spread at best. I get pistachios are expensive, but reducing the amount is not the answer to maximize profit. You could opt to sell smaller containers, for one thing. As far as additions to the butter, try spices, and use honey instead of corn syrup or white sugar. Wild flower or wild honey will both bring their own flavor notes, or you could stick with the blander clover type, or even orange blossom to bring a specific floral note. Anyone with a food processor can make nut butters at home themselves, so what you offer is primarily a convenience and maybe a smoother product. I would suggest you keep the recipe simple, with more nuts, and no milk powder, add spices, and perhaps consider honey as your sweetener. I would not suggest using fewer pistachios in any case.