Do you think there's significance behind Dave breaking Dirk's unbreakable katana when he decapitates him in collide?

Dave’s Symbolism with swords goes deep, however what it means is up for debate. For me, Swords are a symbol of Heroism, but in Dave’s case, he comes to associate Bro as a Heroic Figure- As such, the symbolism of the swords is twisted in his Mind. If Bro is a Hero, then he is not one, because of the abuse suffered by him, he struggles seeing himself as Heroic, and as such can never manage to take the Sword from its pedestal in a normal manner. Swords, and Fighting, come to embody Toxic Masculinity and Abuse in Dave’s eyes, and while he comes to appreciate Dirk, he IS still the teen embodiment of the figure that abused him for so long. As such, destroying the Unbreakable Katana as he decapitates Dirk, is a moment of overcoming his past, one last Fight, one last Slice, to tear through his history of abuse and pain, one last jab at his awful brother, before they can finally put everything behind. Dave becomes, undoubtedly, a Hero, but strikes down the symbolism representing the toxic Heroic Ideals he’s been raised with.

On the contrary, you have Roxy, who instead of rejecting the ideals the Sword embodies, she twists them- Just like how she takes it out of the ground with her Void Powers, no pulling, no breaking, she uses her own methods, and in turn, makes the sword mean what she wants it to mean. For her, the sword is not Bro’s, it’s Dirk’s, and as a tombstone, it comes to embody, she tell us, everyone who didn’t make it up to the end. The will, the hopes and dreams, of everyone, from every timeline that died along the way for them to reach the point they’re at, and as such, while she delivers the final blow on Condy, it’s not her, but the combined strength of every single character in Homestuck that wanted to be happy and didn’t get a happy ending, that’s behind that last strike.