Have you checked that the City Hall and Cathedral are open for "touring" on Sunday if you intended to go inside? I am not going to look them up now, but often civic buildings are closed on Sundays and the Cathedral viewing may be limited due to services. You are probably aware, just mentioning.
I found the Palace quite underwhelming. Given your timeframe and apparent interests, I suspect you might better enjoy the Folk Museum - I agree the stave church is impressive. The rest of the museum is terrific also. Do check the ferry timetable. We took that instead of bus as it was faster. We walked to the Folk Museum from the landing point at Bygdøy along with many others. You could google map the distance. There is a bus from the landing point also but I don't remember the number (my visit was June 2023).
It seems that you were hoping for an overview of the city with your various walking routes. With such a short time and an interest in the Folk Museum, I wonder if you would better enjoy all day at the Folk Museum or combining that visit with something like Kontiki (unique) as suggested above. Kontiki needn't take long. The other museum suggestions above are very good also (my opinion). It all depends on your interests.
The initial route Palace, Fortress, Frogner would have you back-tracking. I assume that is to accommodate closing and guard changing times? To me, it doesn't make a lot of sense. Is that a GPT Chat suggestion? (rhetorical question)
I have not seen the Nobel Peace Center - I left it off our itinerary based on reviews and the better options available. I wouldn't trust GPT Chat to choose the "best" spots if that is where the idea originated. (Of course, this may be a personal interest - but not clear from your post.) I did a quick google ("Oslo itinerary") and noticed several blogger suggestions with 1,2,3 day itineraries. If you haven't looked at some of those, it might be worth your time.
I am a big walker - I've walked many cities on multiple continents. Frankly, I found Oslo one of the harder to get an overview sense of. Bygdøy has sites close together; in another area, the Fortress isn't terribly far from the National Museum; Munch Museum is close to the Opera House. We taxied to Frogner from our hotel near the train station, then walked back past the palace. Many residential areas in between - pleasant, but nothing unusual. If you are wanting to see something unique to Oslo, one of the museum sites might be better.
I found after our trip that I had one fewer day available than I would ideally have liked. We were in Oslo for 5 nights - 4 full and one half day. Lots more to see than I had realized. Good luck planning! There are hard choices to make!!