Good job. It doesn’t matter very much if it is Sentinel-1 A or B. You can even combine Sentinel-1A and B in one pair to reduce the temporal baseline to up to 6 days. What is decisive is the perpendicular baseline which should not be too small. Values over 100 meters as in your example are often more suitable than very short ones because they are more sensitive to topographic variation. Furthermore it is important that there is no rain during the acquisitions because this leads to atmospheric disturbance of the phase. Lastly, images from the dry season contain less soil moisture and vegetation features, which lead to phase decorrelation.
If you use phase to elevation, a external DEM (at best SRTM 1ArcSec) is used to put the unwrapped phase into a vertical context that makes sense. Your unwrapped phase can still contain trends or ramps which superimposing the actual topography but this can be prevented by the selection of suitable images as described above.
As for the availability of data in 2018 and 2019: Sentinel has a nearly global coverage so there should be images, maybe of other tracks. But according to the acquisition plan some regions are observed at shorter intervals than others.
https://sentinel.esa.int/web/sentinel/missions/sentinel-1/observation-scenario