It looks like you are using a masked material and not a translucent one?
This looks more like a z-fighting issue rather then a sorting one
I would set the Z position of the particles to a random value between 0 and 1 to add some offset between each particle so that they are not all on the same Z position
If you were using a translucent material you have full control over the sort priority of each particle, you can bind it to any particle attribute, including the particle’s lifetime
Check out this mini tutorial for more information about sorting in Niagara
https://realtimevfx.com/t/niagara-5-3-sorting-mini-tutorial