Ok.
To clean from within Synaptic, first in the left column, at the bottom part, select "Status", once done, in the left column upper part, if there is such an entry select "Installed (auto removable)" - this entry appears only if some packages can be safely removed -
If there are several in the main part of the window at the right side, select the first, press Shift, select the last, they are now all selected (or select one and use "Ctrl+a") : then right-click on them and use the "Mark for complete removal" option, then hit the "Apply" button located under the menus.
Here is what it looks like at this stage:
Next, you will want to remove the configuration files left behind. In the left column, still with the same "Status" entry, upper part of the left column, hit "Not installed (residual config)".
Select any entry you will find there, with the right-click menu on the selection(s) hit "Mark for complete removal", hit "Apply".
Next, it will be about orphans!
First, install the deborphans package.
Two, at the top menu go to Settings > Filters
Once there look at the following screenshots:
Once this configured,
and you can find out more orphans after you remove the first ones:
You can continue until no more orphans are left.