Norway by car vs public transportation?

Both can be the answer.

I recommend to rent a car in Voss because one-way rentals can become expensive in Norway. Take trains between Bergen and Voss and from Voss to Oslo - scenic.

With a car you will make the most out of your time in that area because you will have light from very early in the morning until deep in the evening.

No car ferry between Gudvangen and Flam but Gudvangen and Kaupanger which allows after another ferry driving the Aurland route back to Flam. Do the Aurland in the morning to have the best light from the back at Stegastein.

Good base for me was always Myrkdalen Hotel. With 2-3 nights you can do
one day: Stalheimskleiva - Flam - Stegastein - Aurland - Kaupanger - ferry (reservation recommended) - Gudvangen - back and
another day: Hopperstad church - Vangsnes - Gaularfjell route - Skei - Fjaerland - back.

Have a good time.

Is there a car ferry from Gudvangen to Flam?

No.

We’ve read the train from Flam to Oslo is beautiful

It is.

Appreciate thoughts on bases and must sees between Bergen and Flam, as
well as car vs public transit.

First, find out where you want to go and what you want to see. Then decide the best option to get there. Also, it's not a question of public transportation or car, often a combination can be the best idea.

A car can give you a bit of flexibility, but driving can also give you more stress. If you decide on driving, make sure you know the laws and all the road signs, traffic tickets in Norway are no joke. A speeding ticket can easily be the equivalent of €1000 or more.

Thank you for these helpful — and fast! — replies! markK curious if you see any benefit to a night in Flam? I like the idea of basing in Myrkdalen and itineraries you outlined.

We stayed in a cottage at the edge of the water in Aurland. There was a grocery store a couple blocks away. You could take a fjord tour of Flam to Gudvangen and take the shuttle bus back to Flam.

Most of our trip was a different route, but I did recently post a detailed trip report of our 18 days in Norway.

if you see any benefit to a night in Flam?

Not really, because it is imo nothing special and fully overprized. The name is only famous because cruise ships can get there. Same like Geiranger. Flam is a very tiny village at the end of a sub-fjord with little things to do - Flambana may be, but large water falls you will see enough there, e. g. Tvindefossen/Trollafossen between Voss and Myrkdalen or with a llttle detour from Voss my favorite Skjersfossen which is part of Hardanger route.

make sure you know the laws

Although I agree to Badger's recommendation but the Norwegian Public Roads Administration does not deliver a simple English translation of the Norwegian traffic rules. Requested since 2018 - also up to minister level - and never delivered.
They just published 6 advises in 5 years!

Thank you all for the very helpful guidance. A few more questions, if you have thoughts:

1) in terms for trading off time in Bergen vs Oslo: thoughts on which to prioritize?

2) it appears the trains from Bergen > Oslo vs Voss > Oslo are different (express vs regional). Is that correct? And any difference in experience, specifically views & comfort)?

3) guidance on how far in advance and where to purchase train tickets from abroad for next summer (early July).

Thank you!

1. Oslo has a lot more and better museums. Bergen's a lot smaller, so you can more easily wander around without depending on public transportation. Bergen gets hilly away from the waterfront, though. Bergen's closer to the fjords.

2. I'm not sure, but I think the deal with the trains is that the section of the line between Oslo and Myrdal or Voss requires seat reservations and has varying fares (buy early for lower prices). Between Myrdal or Voss and Bergen it's more like a commuter line. I think service is more frequent on the western section of the line as well. (Wait for confirmation on this info from others.)

3. I don't know how early Norwegian rail tickets go on sale. I bet that info is available from the Seat61 website. Tickets for the eastern segment of the trip will almost certainly be cheaper if bought well in advance. Check rules on cancellations and changes and consider how certain you are the trip won't be postponed.

A bigger issue than train tickets may be lodging, especially in the fjord area. Booking six months in advance may not be early enough. Flam is particularly difficult. If you decide to rent a car, you'll have more flexibility as to where you'll stay.

The best train fares are available when Norway’s train operator, Vy, first puts them on sale 90 days in advance of the date of travel. So for a travel date of June 1, you’ll want to buy your tickets
at www.Vy.no/en on March 2 which is 90 days ahead. The Bergen line, called the “ Bergensbanen,” that travels to Oslo in 6.5 hours on the fastest trains costs about $70 in standard class when bought 90 days in advance. It’s is a bit less if your trip originates from Voss or Myrdal.
In my experience, the best views are on the southern side of the train— particularly when going through Finse and the Arctic tundra-like scenery that surrounds the highest village in Norway. But you can see both sides of the train out the windows in the space between two carriages where there are no seats, but usually windows facing both north and south. When the scenery is as spectacular as it is on this route, you’ll want to take photos and videos in both directions.
Have a great trip!

1) in terms for trading off time in Bergen vs Oslo: thoughts on which to prioritize?

Depends much on your interests since there is no objective reason for one prior to the other. Oslo is the capital and is much larger, therefore a higher number of attractions and museums, also unique ones like Munch museum. Bergen has different highlights and also a world culture heritage. Bergen is nicer to me and Oslo worth a short visit. As said, depends on your personal preferences.