Hey, sorry I haven’t had time to look at this very closely yet.
I still think I would probably move this all to a custom unlit material since who knows what exactly the volume decal code is doing. But here is some low hanging fruit.
You use acos and atan2. You should try replacing with acosfast and atan2fast. See FastMath.usf for details. In most cases I have found, the fastmath functions work fine for volumetrics stuff but it can be hard to predict where precision issues show up.