Hi Marc,

The first thing to remember is you can not do everything in 4 weeks. As you write Trondheim to Nordkapp is straight forward but from Oslo you have many options. If you look on the individual specific pages for National Cycle Routes and Atlantic Coast there are many options to skip sections using public transport.

Route 9 is very remote and I think for your trip this is not a practical option. Route 7 is nice and direct as you write and easy to jump on train and skip sections if required. However taking a combination of routes to Gol, Vinstra, etc is a more beautiful route to do. If you reach Vinstra and are behind schedule you can jump on the train to Trondheim knowing you have experienced the most beautiful areas between Oslo and Vinstra.