Anke#IRecentlyGrumbled about struggling with the TreeView stuff but wait, TreeViews are deprecated as of #Gtk 4.10 so I tried to get my head around GListModel and ColumnView and I can report a small success as I have managed to create an example working.
#IRecentlyGrumbled about porting my Gtk3 app to Gtk4 and I've started to really make progress and now I'm (unsurprisingly) stuck with the damn TreeView stuff (again) but I'm afraid it's too late to stop now... 