How would i use /fill to fill an area with downward-facing dispensers filled with a bucket of lava?

In terms of using /fill, I don’t think you can do that unless you want to fill the area with lava. I personally made something like this in my world before. First, I actually placed dispensers facing downwards that way I wanted to (so in your case, place 516 downward dispensers), then fill each of them with a lava bucket. I then just used the /clone x1 y1 z1 x2 y2 z2 x3 y3 z3 command in a command block to copy the structure or you can alternatively save and load it using structure blocks and redstone. To prevent mistakes when cloning, try it out somewhere else first so you know where you should clone the structure.