hm, I haven’t worked with EW data so far but if you already terrain corrrected them, you should select “geolocation” instead of “orbit” in the initial offset method.
If that doesn’t work, increase the number of GCPs and the coearse coregistration window.
If still no success, you can select radar > coregistration > stack tools > create stack
Maybe this is already sufficient (given that the terrain correction worked well)