I propose moving
down to D. Not only are they on the really slow end in terms of both startup and endlag, they come with even more problems in practice. The move incorporates a swing...and step. They keep moving forward after the swing which is inconvenient when they're blocked, because they're putting themselves into shieldgrab range after the swing. Marth in particular has this really bad sourspot closest to his body that's negative on hit until ~65% when it starts inducing tumble. You can dash grab or dash attack Marth on reaction before he can act again. But the other sourspot hit (yes, there are two) doesn't have this issue. Lucina has a bit of this issue at very low percents, but hers induces tumble sooner and has only one hitbox so there's no need to worry about spacing. The main draw for Marth is his killing power on tipper, but is it really significant? It begins killing Mario at the ledge at around 130%. Marth's dash attack is notably weaker than his Ftilt, as well as Dolphin Slash. And you have to remember that the tipper is only about 10-15% of the move's range. To compare to other killing Dash attacks, Link kills at the ledge at 75% with the tipper, and 100% with a sourspot. Ike's sweetspot kills at 90%. Roy's at 100% with his sweetspot. Lucas at 120% with his sweetspot. Little Mac at 120% with his sweetspot. And Dedede kills at 65%. All of these moves kill sooner and more consistently than Marth's.
As for
, I think C is appropriate for him. As mentioned, his kills 30% earlier than Marth's (it's comparable to his and Marth's Ftilt), and his sweetspot is more than half of his blade. And despite Roy's backhanded sword style, I think his dash attack hitbox is larger than Marth's. Though it is hard to tell by looking at them since Marth swings... and then steps to make it look like a much shorter attack than it actually is. Plus Roy's sourspot isn't lacking in knockback so much that he could ever be punished on hit like Marth and Lucina could. Plus you'll see Roy's Dash attack in combos. Linking from moves like Nair, Fair, Dtilt, and Fthrow. True combo ranges are small, but it doesn't hurt to try when they have to burn double jump or tech roll to escape.
And Falcon should move out of S. I know it looks like Samus', but if you watch carefully, Samus engages a jetpack that propels her into people far away. If you want a specific late hit, you know when to engage the dash attack in terms of distance. Falcon's late hit is much more precise to land, and doesn't have nearly as much combo potential because of greater endlag, and Falcon's longer startup on moves. Falcon's dash attack is certainly low commitment and has ten active hit frames, but that's all it has going for it. It doesn't lead into high damage combos or kills like other current S tiers do. I think B would be better for him.
Cloud also has to move down to B at least. It has the same startup as DK, but with less active frames, less range, and more endlag. It also doesn't lead into anything or kill.