Have you got no idea of the approx time you want to spend in each land mass?
The European winter will surely impact. would suggest no earlier than April in north Europe. Hopping a train in Europe can certainly help to cover distance if you have a fixed destination/ exit point in mind. Perhaps also in OZ NZ and South Af.
Round the world ticket worked for our recent 6mth tour of Europe with short stop over in South Africa and initial equipment pick up in USA. But we had fixed exit dates/places.
You need a plan but be prepared to change based on what fellow cyclists advise on the spot. We did some wonderful totally unplanned routes in Europe on this basis.
Recommend in Southern Germany Boden See to Koning See following marked bike route. Stunning. Also the Alps. Splugen and Albula passes - stunning.
Check out the Bike line routes - amazing literature including excellent strip maps.
Good luck