For classification purposese, Sentinel-1 GRD data is sufficient. I see no point in downloading, debursting and merging SLC data, to be honest. If wou worked on GRD data you could simply skip the first step and directly apply the orbit file and calibrate to Sigma.
Multi-looking is also not advisable for GRD data because the data are already in ground range.
Did you see this: Radiometric & Geometric Correction Workflow