We are working in Unity3D on an application (actually for VR and MR but the effects are visible everywhere) where we want to fill a room with steam/vapor/fog. Currently we are using a particle system emitting billboards of foggy/cloudy sprites and it looks ok in the beginning. However, when there are many particles it becomes quite obvious that these are billboards behind each other, especially where they are intersecting the walls (see second half of GIF below). The walls will not be visible when in MR, but in the headset you can also see these planes behind planes effects and the intersections with the (invisible) walls
You can see the described effects on this Youtube video.
For hardware reason we are bound to Unity 2020.3 and the built-in render pipeline. Are there ways to work around this effect? Or are there better solutions for volumetric fog given our requirements?