Why would you want to filter and then terrain flatten the image?
The whole intention of generating Beta0 is to supply it to the next step for terrain flattening without disrupting the statistics which will happen if you introduce filtering between them.
Instead, generate the Beta0 and terrain flatten it so that completes the Radiometric part of the processing and then speckle filter the image before proceeding with the Geometric part which is the Terrain correction.
This is exactly what I discuss with Andreas,
here: Radiometric & Geometric Correction Workflow