I've never seen the Flamsbana called "way overrated" or even "overrated". What it is, is expensive on a per-mile basis. It would be possible to have such horrible weather (rain/fog) that the views would be severely affected, and that would be an expensive bummer.
Norway has tons of magnificent scenery; you'll see a lot of it while you're driving around. I suppose the Flamsbana isn't essential as long as you have other opportunities for elevated views, looking down on fjords from above. One option would be the Stegastein viewpoint not too far from Aurland (I haven't been there). To me, this is a bit like skipping the Naeroyfjord ferry trip. I'd be questioning that if you were not going to see the Geirangerfjord, but you are, so I think you'll get enough of that sort of experience.
I have been to Alesund and liked it a lot, but I am uncommonly fond of Art Nouveau architecture. I am not going to push that substitution since you mention it would mean considerable extra driving. In addition, you may want to work in a stop at one of the stave churches you'll be near, and there's the aforementioned Stegastein viewpoint. I managed to get to the Borgund Stave Church while I was staying in Flam and recommend it highly. There's a small stave-church museum in the visitor center there, and I think there's a food source as well. I was tied to a bus schedule and had inadequate time to do anything other than see the church itself.
