Cascade - Translucency sorting

On request by Alexander Paschall I am posting this here.

I created a black-hole effect for a client:

(mp4 version:

that black sphere is transparent when growing, then is swapped to opaque for the remainder. thing is, as long as its transparent, all the stuff behind it is showing trough, and stuff in front of it behind it. I tried changing the sorting modes, but none of the results is satisfying. I also tried having the spheres in a separate emitter and combine them in a blueprint, again swapping sorting orders in the hopes the order of translucency would be “correct”.

Currently the client is happy with a version that uses a mask instead of transparency, which of course is more pixelated.
which… doesn’t look too bad:

But if its possible, I’d like to have the sphere transparent for additional visual sweetness.