To ensure it includes the PATH and PYTHONPATHS associated with Slicer extensions are also available, the additional launcher settings for the associated revision of Slicer need to be loaded.
In the Slicer launcher, that mapping is described by the following :
[Application]
name=Slicer
revision=28975
organizationDomain=www.na-mic.org
organizationName=NA-MIC
This ensured setting file like /home/jcfr/.config/NA-MIC/Slicer-28975.ini is found by the launcher.
If you manually add a similar section to the settings of PythonSlicer, could you check the extension settings are found ?