It just seems like a very general statement, or a broad goal, from pirate to pirate. It's like, they pretty much all want treasure, but treasure isn't their main or only dream, let alone a dream at all. Like I said then, searching for One Piece doesn't have to equate to becoming or wanting to be the Pirate King, and just because Luffy says he was looking for it as well doesn't mean that was Ace's primary goal. I mean, hell, what did Usopp ask Rayleigh the first chance he had? Usopp's ambitions aren't to be the Pirate King, but to be a 'brave warrior of the sea', yet his first question to Rayleigh is about the location of One Piece. All I was saying then, which is what I am saying now, is that Ace could have a different goal in life (which he did). A secondary goal, a means to an end, or anything else in between wouldn't matter much, so long as he held true to his real or original intentions. Which, as far as I can tell, he did.
I dunno.
Oda has cast becoming Pirate King as a nigh on impossible goal that most people would laugh at you for. To become Pirate King you need to find One Piece, and that's what ace apparently set out to do when he left the island.
Unless it's something roundabout like wanting to find about the history or somesuch, but since we've yet to see any interest from ace in anything of that nature, I'm still going with Pirate King being Ace's goal.
Of course, it could be as simple as Oda offhandedly writing that Arabasta bit as "ace is a pirate and all pirates worth their salt should be looking for one piece" and having since forgotten about it(it was almost a decade ago).
As for usopp, what else would he ask Rayleigh - "can I have some courage"? He's a man who knows where and what one piece is, not the wizard of oz.
But yeah, I agree that Ace did set his goals lower then I'd thought, and achieved his original dream well before he died.