There are different forms of fats - nuts being plant based contain unsaturated fats which is considered a healthy fat - can help lower cholesterol and has other health benefits. However even though considered healthy they still need to be fed in moderation.
With pistachios I buy them from the baking aisle as they are unsalted and unroasted. I give them in their shells so they have to work a bit harder to get the out.
All protein are typically higher in fat. Chicken is lower in fat being a lean meat but other Lower fat protein source include dried grasshoppers, or freeze-dried shrimp but as your Syrian doesn’t like mealworms then she probably won’t like these either.
I use these freeze dried chicken treats -
https://www.petsathome.com/shop/en/p...cat-treat-27gm. You can use freeze-dried chicken from other species as long as it 100% chicken which no added preservatives.