Obviously they WILL trim the dialogue, but it's not the length of the dialogue that makes Homestuck, it's the quality, and animated mediums have the benefit of being able to develop their characters and worlds through means other than dialogue - primarily their visuals, but also for instance other elements of sound that we don't experience through written dialogue alone, like tone. This is always how adapting the written word to the screen has worked, since the screen was invented, and Homestuck is not in a position to uniquely break these long held conventions. I mean, presumably it WILL break a lot of the conventions of the medium, that's what this thread is about - but it'll do that on purpose, because breaking conventions is much of what Homestuck is about.

Spending some large number of seasons adapting Act 6 is inevitable in any case. You need to remember that Act 6 is the vast majority of the comic, and it's where everything happens! Compared to Act 6, the first five are basically all setup, and imo if Hussie knows what he's doing (which they almost certainly do) then they'll try to get through that setup in as few episodes as possible to get to the real meat of the story.