Sometimes, the topographic phase is modeled, removed before unwrapping of what remains and later added again. This is said to improve the unwrapping.
Topographic phase is removed for differential interferometry, for example for the detection of subsidence between two images. For DEM generation, you are highly dependent from the variation of the phase which originates from topography. How else would you describe it then without it?
Please update snap and use the SRTM 1Sec in the Phase To Elevation module.
No export needed, the img files in the data folder are already GIS compatible and basically ENVI raster files. But your last step has to be Range Doppler Terrain Correction to put the data into a coordinate reference system.
It’s worth trying the other SRTM, yes.
About the fringes: much depends on proper coregistration and use of metadata. Did you apply orbit files as a first step?
The SARscape image looks like it’s already unwrapped (no more fringes)