if you want to use interferometry, you skip the calibration to Sigma0 and directly start with the co-registration.
If you simply want to create a stack of backscatter time-series, you calibrate, geocode and then stack them (Create stack). If you want to go sure, you can coregister the geocoded products (Coregistration). This takes more time but especially if you use images of ascending and descending orbits the quality of the alignment of the images is better. Just to go sure you select “geolocation” as initial offset method.
Once you have a stack of multiple images, you can decide if you want to apply a multi-temporal filter. But especially if you are aiming for subtle changes, you need to check that these changes are not elimited by the filter. They are adaptive but there are also reports where they caused the loss of relevant informaiton.