The E6 is the main road going north-to-south in Norway. All heavy goods traffic, all the lorries carrying supplies, most of the rental campers and cars with drivers scared of small roads, go on the E6. And outside the southern populated areas, it is a twisty dual-lane road. Even with the acceleration of a sportbike, you just don't have the visibility for large stretches of it, and when you do, there can be solid traffic both ways.

It's just infuriating, and easily avoided in favor of much more scenic and pleasant Rv-roads most of the time.

Helsinki to Karasjok is doable in two days, but my advice would be to look into the car train from Helsinki to Rovaniemi or Kolari. I'd say the western highway from Oulu/Kemi via Kolari, following the Muonio river and Swedish border, then turning off at Enontekiö to head towards Alta and Nordkapp from there, is more fun than the eastern highway from Rovaniemi to Karasjok - but both are essentially fast, straight, and boring. But watch out for the reindeer, they WILL be standing right in the middle of the road after a blind crest.

If you're going to Karasjok, you have to stop by Nordkapp, for the bragging rights. If you spend the night around Inari/Ivalo/Karasjok, you can easily get to Nordkapp and back down to Alta/Hammerfest in a day, to spend the night there.

Again if you're going south to Mo i Rana, definitely go to the Lofoten islands. Try to spend the night in Harstad (a very nice town with a historic center, unlike boring industrial Narvik, and also accessible without ferries). Or if you spend the night in Tromsö, take the Senja Island route and Gryllefjord-Andenes ferry and go down all of Lofoten, then take the late fast ferry from Moskenes to Bodö.

Trollstigen and Geiranger are both great, but don't put much stock in Kristiansund (the city next to the Atlantic Road), it sucked. I feel personally the Atlantic Road is skippable - it looks best in photos. And it's a huge detour via otherwise unremarkable (for Norway) roads. Go from around Trondheim via Rv60-family roads through Andalsnes to Trollstigen (much better done north/east towards south/west - you approach the switchbacks heading up, then have long sweeping curves on the other side going down to Geirangerfjord) and from there again on Rv60-roads towards Lillehammer.

This is where you might as well pick up the E6. Take a ferry from Oslo to the top of Denmark, blast down the E45 and into Germany, autobahns all the way through Hamburg-Bremen-Rotterdam and back to the UK.