NIN starting Bergen

We would prefer starting in Oslo and going west. We caught a connecting flight home from Bergen for about the same price as a flight home from Oslo.
Bergen is one of those places we would like to return to. The people are tall, slim, blonde and simply beautiful inside and out.

I think you would be missing out on the Flam railway descending to the fjord area. Supposed to be beautiful train ride. We go this summer from Oslo to Bergen. Otherwise, the fjord area will look the same :/
When I used to sell snorkeling trips in Hawaii, I used to always tell the clients that the fish don't care what kind of boat you come on! However, there are lots of different boat options!

You can do NIN either way (Bergen to Oslo or Oslo to Bergen)
Starting in Bergen...take the train to Voss...bus from Voss to Gudvangen...boat from Gudvangen to Flam...train from Flam to Myrdal...train from Myrdal to Oslo.
You can get some of the train tickets ahead of time through the Norway Train system (NSB). You can actually call them to help you book train tickets, if you're having difficulty with their website.

There are lots of options to do this depending on how much time you have - I have started to do some of my detailed research for our upcoming trip this August. I too will be doing this going from Bergen to Oslo but I'll be spending a couple of days in between driving around.

Starting out on the train from Bergen to Voss. You can check timings at www.nsb.no - scheduled only seem to run to April at the moment. Key trains depart Bergen at 6:52, 7:57, and 8:43 taking between 1:10 - 1:20 for this journey.

Then take the bus - this thread had some useful links - https://community.ricksteves.com/travel-forum/norway/bus-from-voss-to-gudvangen

From Gudvangen you have a choice of ferries - to Flam/Aurland or to Kaupanger which is where I am headed. Will then pick up a rental car and drive through Stryn, Hellesylt, Geiranger, Andalsnes, Sogndal and return to Leikanger where I'll grab the fast ferry to Flam and will then take the Flamsbana to Myrdal and then the train to Oslo. It doesn't look I can make the earlier train into Oslo so will end up reaching there after 10 pm on the late train.

Here are the ferry schedules I found
https://www.visitflam.com/globalassets/rutetider/rutetider-2018/fjord-cruise-naroyfjord-2018.pdf
http://www.fjord2.com/about-the-car-ferries-on-the-naeroyfjord.html
https://www.norled.no/contentassets/dda9d4d3d61d4b158bc23c5692747de5/bergen-sogn-flam-bergen-2017-2018-oppd..pdf

The www.visitflam.com website has an easy download of all the schedules including the ferries, buses, and flamsbana that will be useful in your planning.

Variation on the above themes: My son and I are going in May and will start in Bergen. Our tentative itinerary from there: ferry to Balestrand, overnight there, ferry to Flåm, train to Myrdal, train to Oslo.

You mention you are picking up a rental car in Gudvangen, where are you returning the car.