I've only recently started writing for Dave, as I am relatively new to Homestuck fanfiction writing. But generally, I tend to make his speech incredibly lax, where he only says "big" words if it's to be funny or silly in some roundabout way. I feel that it helps to be more direct and to the point when writing Dave's speech because he isn't the type of person who is vague in his meaning.

I also don't really curse a lot, and I noticed Dave curses but it's not that often and he'll only say words like 'shit' and 'fuck' when he's talking for a long time. Or when he wants to get his point across.

"his typing quirk or typing pattern is like this all lowercase with no punctuation whatsoever"

I have broken some rules, though, and used exclamation points when he's really angry in some of my fics just because I wanted to get the tone across lol. And that can be kind of hard when he has such a flat tone. As for how to write his personality, you should probably just focus less on trying to make some funny scenes with him, since the comedy will kind of just come naturally anyway, with how absurd his situations tend to be. It just depends on the fic you're writing IMO.

If you're writing a really serious fic about a very serious topic, then I think it's totally fine to have Dave care about the situation since he does care about difficult problems. He has a flat tone, but this does not mean he is unpempathetic in any way to other people.