My current game is homebrew mix with adventure sites from The Spire of Quetzel. But the principle I use for the adventure locations and the artefacts are the same - legends. Each has nice story that I try to provide to the players in one way or another, depending where they go and with whom they speak. I try to shape the story according to their actions and interests, so they have stimuli to go this way.
I am doing it the same way and also give my player information about the artefacts a long time before they will actually find them.
The free-form, location based nature of the Raven's Purge and the lack of solid timeline and suggested course of actions is indeed weak part in the campaign.
I just want to point out that these were the specifications for writing the campaign. Like it or not
I think the free-form is a big strength of the campaign and even though it is a little bit more work, I really enjoyed preparing the campaign. But as I said I quite struggle with the key players. Arvia of Crombe wants the Hanging Stones for her research in stone magic. Zytera, Zertorme and Kartorda want the Crown to crown themselves. Empress Soria also might have plans for the crown.
I wonder how one could portray the hunt for the crown. How can the other key players/factions be a competition for my players?
I think the easiest way is to replace some random encounters with artifact hunters. Roving bands of adventurers, possibly allies, possibly enemies, hunting for X artifact. Maybe the first group is after a different artifact, maybe one the players don't even know about. Later, if they have one, maybe a group of bounty hunters come after them. None of this has to be on a time line, just having a few encounters letting them know that others are around, looking for the items.
Another possibility is have the players encounter a group that has discovered one of the artifacts and is racing back to whomever wants it. Let the players decide if they let it go, try to steal it, or help them return the item.