you can check if there are preprocessed interferograms of your area in this portal: https://comet.nerc.ac.uk/COMET-LiCS-portal/
They are a good reference regarding how good ones look like. Click on a tile, then “Link” then “interferograms” and select a date. There are png files for comparison.
The differences regarding the DEM do not cause shifts, but you have to level all displacements at one stable point. We had a discussion on this here: Different results in the study of land subsidence using the Sentinel-1 satellite image