Recommendations for first time visitors

I will travel to Norway with two friends. We will land in Oslo on October 19th and will departure on the 25th, so we must be back to Oslo by that morning the latest.

We searched around and listed the following sketch:

– Go from Oslo to Bergen (passing through Flåm and Myrdal) by train and ferry

– From Bergen, fly to Tromsø and try to see the Northern Lights

– Then from Tromsø back to Oslo

We are a bit lost about how good is that plan, and how many days to stay in each place.

We saw that Oslo to Bergen can be done in a single day and it is a trip by train first and then there is a ferry through a fjord, but we don't know if we should stay a night in Flåm or Myrdal or other stop before Bergen, nor if we should stay in Bergen more than one night. We also wonder if it would be better to first go to Tromsø, then Bergen and then do the trip to Olso from Bergen.

Our main idea is to see the northern lights (if we are lucky) and some nature (e.g. fjords), we are fine to miss spots of Oslo.

Do you have any recommendations or guidelines for us? Anything is welcome, e.g. don't stay in X place, take Y train/ferry, don't miss Z thing, you should go to W instead of …, etc.

Another question we have is: are there frequent flights trains between these destinations? Or should we fix dates and reservations in advance?

Destination aside, are there things we should know that are normally a surprise to visitors?

E.g. do you accept cash in hotels/restaurants/tours? or do you only accept cards? Are there foreign exchange offices to exchange EUR or USD for NOK?

Thank you all in advance!