we may never know
We do actually know!!!!! When he started HS, it was almost completely reader driven. (Heck, all Beta Kids+ Beta Trolls names are reader suggested names) Commands were suggested by the reader base, not unlike PS. By Act 3, that began to dwindle, but he also said he had a more clear idea where the plot was going.
But don’t be fooled- readers still pushed the story in new and exciting directions (hello, Act 5. Part one at least.) So he kind of just spontaneously decided/reader pushed him to introduce us to all the trolls and give them a story as well.
Also, his trademark sidetrack/plot detour is pretty much the work of a man winging it as he goes along.
In Doc Scratch’s apt, he admitted he lost his vision for where the story was going, which shows a fair amount of planning. He’s also explicitly stated the End of Homestuck has remained rather constant in his mind since day 1, so he’s been working towards whatever that is since April 2009. That’s a LONG ASS TIME to be preplanning.
So it’s both, which, in part, is why Homestuck is so addictive.
It takes a million twists and turns but almost never do you not see that it’s building to a solid, concrete climax.