I agree with the most.
But.
I personally don't think so it's connected to Secrets of void century.
Like aqua laguna confirmed all Blue exists somewhere.
There's no secret to it.
All kind and special kind of fishes are here. Sanji wants to cook it that's it.
Its simply a place where all 4 seas meet.
And is on grandline.
He just needs to find where is it. It really ain't that difficult. Compared to other straw hats.
Also I think you missed brook. Isn't his dream to meet Laboon. How is that difficult?
The All Blue is probably the most vague thing in One Piece so far. As far as I remember, it was just mentioned in the Baratie Arc and never again (besides some fillers in the anime). No more info about it whatsoever. You're right, it could be literally anywhere in the Grand Line (it's the only hint we have, as not even this is confirmed), but I doubt it would be as easy as getting to a place nobody has ever been, otherwise it would've been discovered (publicly) already. Does it even exist in the current world's structure?
The thing about Brook is that he promised to meet Laboon after sailing all the way around the Grand Line. Yes, he could just go back to the Twin Capes via Paradise, but he put himself that self-imposed goal, to keep his promise to Laboon. I commented about him in my first paragraph, 'cause he's 1st place on the poll right now.