It’s all about how orbit files work.
InSAR requires accurate information on the position of the satellites at the time of acquisition. Orbit state vectors in the metadata don’t have continuous position information, but at regular intervals. If the image was acquired between position x and y, the method of orbit interpolation determines how the oposition in between is estimated.
Sensors such as TSX or Cosmo SkyMed have very accurate information so there is usually no problem in high interpolation degrees and increasing them potentially leads to even better results.
In other cases, like ERS for example, orbit information can contain noise so higher interpolation degrees can over fit these false positions and make results worse. Accordingly, lower degrees can level out these errors.
To make it short. There is no ideal setting which works for all, therefore SNAP allows to adjust these parameters, but their effects have to be understood.
The ramp in your interferogram indicates bad coregistration quality. Have you tried loading both products separately and coregistering them yourself?
Please also have a look at other topics discussing ramps in TanDEM-X data
I’m not a professor by the way but thank you for your politeness.