Gain seems a bit low.
Also the noise floor you see is after the gain of the SDR, so it’s not an absolute thing.
As only the SNR matters … well a low noise floor after reducing the gain a bunch doesn’t really give you anything.
If you have the antenna in clear view of the horizon and would expect more due to your terrain, think about how lossy your coax is and maybe also consider a filter.
Terrain limited reception: GitHub - wiedehopf/tar1090: Provides an improved webinterface for use with ADS-B decoders readsb / dump1090-fa