If you want to use a self-compiled GDAL with QGIS packages, you have to use exactly the same GDAL version that the packages are compiled against. For bionic (Ubuntu 18.04 and Linux Mint 19), You can find the version in https://qgis.org/ubuntu/dists/bionic/main/binary-amd64/Packages and https://qgis.org/ubuntugis/dists/bionic/main/binary-amd64/Packages if you search for the gdal-abi dependency.
Currently this is GDAL 2.2.3 in the main Ubuntu repo and 2.3.2 in Ubuntugis unstable.
https://trac.osgeo.org/gdal/wiki/ECW suggests to use the ECW SDK v 5.4 with GDAL 2.2.4 or newer, so you have to use the ubuntugis ppa and repo, or self-compile QGIS.
QGIS 2.18.28 from the ubuntugis unstable ppa (without using any qgis.org) installs fine along with the ECW driver, if you copy the libgdal files from /usr/local/lib to /usr/lib.
Upgrading to QGIS 3.4 and QGIS 3.6 from https://qgis.org/ubuntugis-ltr bionic or https://qgis.org/ubuntugis bionic returns a working version of QGIS 3.x with ECW support.