Maybe a few points:
Coherence stronlgy depends from the land cover. Much vegetation leads to decorrelation between your images and low coherence. Where is your data located? Can you post a screenshot of your coherence?
You can increase the number of GCPs for the coregistration. This could lead to better results at some locations. Also try to use a second order polyonomial and trucated sinc interpolation. These things won’t increase the overall quality of the data but baybe lead to smaller improvements.
I don’t think filtering before coregistration makes sense. Maybe you also try also increasing the window sizes.