I used the Astrophoto mode on the Samsung Expert Raw camera app, which you can get on the Galaxy Store. 3 mins of exposure.
There's also an Astroportrait mode to take a picture of a person and then take exposure shots afterwards. Excited to try it sometime.
Looks nice! And that's actually light coming from the window of our cabin. There were some dim lights outside as well, so they very well could have caused the graininess.