All of Rick's suggested itineraries allow inadequate time in major cities for me, and this one is not an exception. Perhaps it's the right pace for you--especially if you aren't much interested in the many museums found in the capital cities, but maintaining that pace for 21 days is different from doing it for 10-12 days.
The one specific thing that sticks out at me is the route between Oslo and Bergen. I wouldn't just go to Flam or Aurland by train and Flamsbana and then take the express boat through the Sognefjord to Bergen the next day, which is what the sketchy phrasing of the linked itinerary suggests. Instead, I'd continue with the classic Norway-in-a-Nutshell route beyond Flam/Aurland, as follows:
Naeroyfjord ferry from Flam or Aurland to Gudvangen (highly scenic).
Bus from Gudvangen up to Voss (also very scenic).
Mainline train from Voss to Bergen.
In case it matters, just about everything in Norway is quite expensive. The exception is food in supermarkets. Maybe also Oslo hotel rooms in August when business travel is light. I found Norwegian hotel breakfasts often exceptionally good, which is a fine thing, since you may not have an option to book a room without the included breakfast.
Lodging in Flam is limited and demand is high. That's a reservation to make as soon as you have dates pinned down. We've had reports of sellouts on some parts of the Nutshell route--I think primarily the mainline train from Oslo to Myrdal and the Flamsbana from Myrdal down to Flam. I don't believe that is an immediate concern; in fact, I don't know that August train tickets are even on sale yet. However, you'll want to buy those tickets well ahead of time. You can monitor the vy.no website to see when August tickets go on sale.