render all the particles with same transparency

You can achieve what you want using two different methods.



1) Only with Visual Effect Graph, you can render your effect using two outputs, a first that will write only depth but not alter color. In order to do that you can use an additive blend mode, write 0,0,0 so you don't alter color, but instead enable depth write. In the other one, you render the exact same particles, as alpha blend and make them revealed by the depth. This is a two-pass that would look the same as render them as opaque, but it enables you setting a constant opacity. I have attached this graph (SingleSpheres.zip) so you can take a look at it.

2) Another more technical solution that could help you achieve something interesting in your screenshot: I can see that you give your particles a metaball-like blending. You could use a HDRP Custom Pass in order to blit radial gradient sprites into an offscreen render target, then resolve this render target using a threshold (see screenshot below) and render it fullscreen at a given opacity.

 

It's working because only one particle per pixel will be drawn because of the depth test.

Another solution you can do if you dont want to write depth for this effect:
Use a shader graph to fetch scene color and apply blending in shader.