You should know that due to atmospheric-artefacts in repeat-pass InSAR the quality of your DEM is likely to be much lower than for example the freely available SRTM or ASTER DEMs.
If you want to make high-quality DEMs from SAR data, you should look into getting the TerraSAR-X/TanDEM-X bistatic pairs from the DLR. They acquire some pairs of SAR images with the two satellites working simultaneously to create what they call bistatic pairs, which are processed to co-registered SLC images (CoSSC). Because the two images are acquired at the same time, the coherence is very high and there is no change in the atmosphere that causes errors for repeat-pass InSAR like you would get with Sentinel-1 or repeat-pass TerraSAR-X. The disadvantage of the bistatic pairs is that I don’t think that SNAP supports processing them.
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I made a DEM with bistatic TDX data with SNAP once and it worked well.
Interferogram
Comparison between hillshades of TDX DEM (left) and SRTM (right)
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That is great news! I did not know that was possible in SNAP.