In the last three posts the first image shows normal particles and the second shows soft particles respectively. So as a conclusion, the disappearing soft particles with greater camera distances came from the depth not having enough precision.
I'm happy now with the visual result of the particles, looks very nice
I'm thinking now abut trying out different fade functions. At the moment I use the scaled depth difference. This value is then multiplied with the final particle color.
If somebody has a helpful link or can recommend a function, please let me know