That's a really heavy simulation for me to run...
Hmmm... I was expecting Pyro could work under the Volume Builder but it doesn't... I tought I could add an SFD Smooth to it ...
I only have a terrible idea ... mesh it with the Volume Mesher, lower the Voxel Range Threshold, make it a child of an other Volume Builder, apply the SFD Smooth and then render the VDB. It's a good thing you have a single color smoke.
Hopefuly this will eliminate the circles because if it doesn't you'll need to put it under the Remesh...
Maybe the safest method is a post effect blur...