Hi Olivier, you are in for a treat with never-ending beauty on every road! You can certainly do it South to North and will probably have more tailwind too!

Things to note

1. From Geilo you have over 20km on the main road 7 to Haugastøl. You could take a train and skip that section. It’s not too busy but expect cars to fly past.
2. from Flåm you will need to take two tourist boats to reach the other side of Sognefjorden. There is some info on the POI map of Fjord Norway.
3. The climbs heading north are going to be a suffer-fest. There are many to do one after the other and it can be difficult to stay mentally strong with a big climb every day. Factor in rest days to break up the journey. But also try and have rest days on unsettled weather days. If the sun is shining you want to try and push through and do the climb as the views will be magnificent.
4. Try to cycle the E39 section early in the morning or late in the evening. It is safe to cycle but there are many cars during the busier times.

The bottom line: If you are doing 60km a day then you should feel okay to manage the climbs (your elevation gain per day should be manageable). The climbs do not have crazy steep average gradients and are all on paved roads. I say go for it!