About the Dreadnought destruction:
In the moment it isn’t heroic. There’s a shot of Poe looking at Paige’s bomber falling into a fiery inferno and he does a sad face.
And then while everyone’s celebrating on the cruiser, Leia looks at the death toll and sighs.
Stuff like that is only apparent in retrospect. In the moment, it seems like Paige’s sacrifice was tragic, but necessary to destroy the Dreadnought. Because of that, when Leia slaps Poe in the face for it, it feels jarring.
Finn’s self sacrifice wasn’t needed because it wouldn’t have changed anything.
But Finn had no way of knowing Luke was about to show up, and neither did the audience. When Finn tries to slam his skimmer into the cannon, it seems like the only way the Resistance can win. So when Rose stops his suicide run and tells him to “save the things you love,” which is what he was trying to do in the first place, it feels jarring. Unless Rose has the ability to predict the future, there’s no way what she did was heroic.