I wouldn't do a Sognefjord cruise from Bergen; I'd spend my water time in the narrower parts of the fjord, most definitely taking the Naeroyfjord ferry from Gudvangen to Flam. That's what you don't want to miss, and the bus from Voss (on the main railroad line) down to Gudvangen is also very scenic. I don't think the part of the train line between Bergen and Voss is particularly special; it's the least scenic of the five segments of the (overall extremely worthwhile) Norway in a Nutshell route. So substituting the express boat ride for the train trip between Bergen and Voss wouldn't disturb me. It's the other stuff you're missing that seems (to me) a mistake.
Norway in a Nutshell is one case when the hype is justified. Make substitutions if you have to, but understand that you are probably trading down from a scenic point of view unless you have a lot of time to explore.
I haven't been to Balestrand. I can well believe it's more attractive as a village than Flam (not a high bar), and it's much less touristy. I tried really hard to figure out a way to make it part of my own itinerary in 2022. It's just that Balestrand is inconveniently located on the north side of the Sognefjord while the easy-to-link-together scenic public transportation is on the south side of the fjord. Or rather, Balestrand is much less touristy because it's on the wrong side of the fjord for public-transportation users.
From what I can gather, Balestrand is great if you have time and a rental car, because there are beautiful roads to take advantage of on the north side of the fjord. Buses are so infrequent (sometimes just once a day) that you need a good bit of time if that's how you want to get around; a rental car eliminates what is otherwise a really significant impediment to staying in Balestrand. People who've stayed in Balestrand universally seem to like it, but I only remember one report here by someone who stayed there and traveled around by bus. I think she was there for more than two nights. I'm not disputing that, in isolation, Balestrand is "better" than Flam. However, for most travelers without cars it is quite inconvenient. And Flam is not a bad place; it's just not charming. The little Flamsbana museum is interesting for an hour or so, and I think there are some nice walks possible, heading back away from the fjord.
The Rauma railway is great. There are always a lot more good options than we have time for in Europe. In Norway, especially, I think the issue of balancing city sightseeing and gorgeous rural scenery comes into play. I'm confident you'll enjoy whatever you do (aside from paying for it!).
I like Helsinki a lot. I've spent about 12 nights there in the last two years. But part of that is being absolutely nuts for Art Nouveau architecture (lots of it in Helsinki, which isn't a really old city) and wanting to go to every art museum in town. For a more typical visitor with five nights available, I'd absolutely recommend spending a chunk of time (beyond just daytripping) in Tallinn. For most folks, Tallinn will have more of a "wow" factor than Helsinki. Ferries make the trip easy. Tallinn really needs at least two full days as far as I'm concerned; in addition to the large medieval center, you have the very interesting 20th-century history to explore. I think day tripping to a place like Tallinn is a real shame.
I didn't spend all my Finnish time in Helsinki. There are pleasant places you can day trip to, but in my view they pale beside Helsinki, and there is just no comparison between Tampere/Turku/etc. and Tallinn.
