QGIS installation vanished! Re-installation attempts produce dependency-errors

In this specific case, you should remove the ubuntugis repository.

Then downgrade gdal (from 2.3.2 that came from ubuntugis) to version 2.2.3.

sudo apt install gdal-bin=2.2.3+dfsg-2 libgdal20=2.2.3+dfsg-2 libgdal-dev=2.2.3+dfsg-2 libgdal-java=2.2.3+dfsg-2 gdal-data=2.2.3+dfsg-2 python-gdal=2.2.3+dfsg-2 python3-gdal=2.2.3+dfsg-2

The install qgis from qgis.org/ubuntu repo:

sudo apt install libqgis-analysis3.4.2 libqgis-app3.4.2 python-qgis qgis qgis-plugin-grass

If you want to install saga, well, you also have to fix a minor problem in the version names of the opencv packages.

Install saga with:

sudo apt install libopencv-flann3.2=3.2.0+dfsg-4ubuntu0.1 libopencv-highgui3.2=3.2.0+dfsg-4ubuntu0.1 libopencv-ml3.2=3.2.0+dfsg-4ubuntu0.1 libopencv-core3.2=3.2.0+dfsg-4ubuntu0.1 libopencv-videoio3.2=3.2.0+dfsg-4ubuntu0.1 libopencv-imgcodecs3.2=3.2.0+dfsg-4ubuntu0.1 libopencv-imgproc3.2=3.2.0+dfsg-4ubuntu0.1 saga-common saga python-saga libsaga libsaga-dev You should end up with QGIS 3.4.2, GRASS and SAGA.