We (59 and 72) have recently returned from a 4 week trip to Norway. We travelled exclusively by train/bus which was a safe, clean and efficient way to travel around. Every Norwegian we met spoke perfect English and was very friendly and helpful. Norway has a hiking/camping culture but if you are not used to hills it can be pretty challenging.
We spent time in Bergen and Stavanger and did a really nice fjord cruise out of Stavanger to Lysefjord and Preikestolen (pulpit rock - it features in one of the Mission Impossible films). Very nice waterfall on the Lysefjord cruise that the boat backed up to and was within three feet of the water. A highlight of our trip was getting from Bergen to Stavanger by bus on a really spectacular road route that featured two ferry crossings.
The resistance museum in Bergen at the fortress was really well organised and free. We ate dinner one night at the fish markets which offered reindeer, whale etc. in addition to seafood like lobsters and cod.
We didn't do Norway in a Nutshell as I was recovering from COVID and didn't have the stamina for such a long day. Maybe next time.
We also went to Alesund and on an all day Geirangerfjord cruise from Alesund. Geirangerfjord was spectacular, even on a very misty day. There is a waterfall walk from the ferry dock in Geirangerfjord where you are walking almost in a massive waterfall. Spectacular but damp.
Also went to Trondheim where Nidaros cathedral and an old fort that was used to imprison resistance fighters in WWII were highlights.
We spent a week in Oslo, staying in Grunerlokka area. We had hoped to visit the Viking museum but it is currently closed for renovation. The Munch museum, Vigeland park and castle/fortress were very interesting. The best view in Oslo is from the sculpture park Ekebergparken which is accessed by a thrilling tram ride that felt more like a rollercoaster than public transit!
On Norwegian cuisine - it was disappointing. We are used to high quality seafood and found Norwegian cod a little boring. Eating out in general is extraordinarily expensive, and we found the limited access to wine very irritating. In Oslo, we rented an apartment but could not find anywhere close to us to buy a bottle of wine to take back to the apartment.
If you do rent a car then be prepared for confusing parking restrictions. In all of the places we stayed having a car would have been a problem. Most hotels in the historic centres do not have parking and you will need to park on the street.
With your limited time I would not try to squeeze in Olso. Confining yourself to the western fjord areas would work well. I suggest Stavanger, Bergen and Alesund as nice places to stay.
If you do hire a car be aware that Norwegian drivers do not speed and seem very patient about waiting to overtake slower cars. The roads were excellent and there are many rest areas with a toilet and picnic table.
Enjoy your trip!