Best options departing Dubrovnic

I'm not understanding your itinerary.
Where does the car hire fit in? You mention using trains, but also car on arrival in Europe.

We would perhaps use a hire car from Venice airport to Ljubljana. Been to Venice 3 times(skip). Would like to arrive Düsseldorf by air, by train visit cathedral in cologne, stop in Dolomites ( Bolzano) again enroute to Venice and Ljubljana. We’ve seen most of Germany other than cologne. Quick visit to Athens if possible on way back to US from Dubrovnik at end of RS Adriatic tour. Not smooth I know.

Drop off in a different country is VERY expensive. Check before you commit to that idea.

We took train to Ljubljana for this tour. We left from Salzburg, but you could leave from Munich. This route was gorgeous and no driving. You could take train from Cologne, though it’s a lot of train travel if done all at once. Going to Cologne, then flying to Ljubljana or Venice is another option.
Have fun planning.

Consider GoOpti, a Slovenian van transfer service, to go from Venice airport to Ljubljana. I have taken them twice. The RS travel books recommend this service. The earlier you make reservations the lower the price.

Thank you for the van info. We would like to revisit the Alpe de Suisse near Bolzano on our way from cologne to Venice. We had a lovely time there on our own a few years ago. Bolzano is easily reached from Salzburg so I’m assuming it is easy from Cologne/Munich. My biggest concern is leaving Dubrovnik; limited air oppties AND would like to pop into Athens before heading home. Anything you can add about departing Dubrovnik via Athens would be appreciated. Most departures head west or north by air from there. Thanks.

We would use train from Cologne to Munich to Bolzano.

To Venice too.