I think it depends on how serous you are about it being as sandboxy as, say, Minecraft, or just giving the feeling of being a true sandbox. Most "sandboxes" I see in D&D, even ones like what you describe, are rather more like a sequence of DM-developed adventures. The DM furnishes the hooks the players can pull on. It's a menu written by the DM, the players just select their preferred dishes. The players participate (perhaps even more than Hussar permits--I think it's still quite possible for the PCs to choose the order of the adventures undertaken, even in D&D), but with how D&D's structure and mechanics work, it's still entirely the DM designing a jungle-gym for the PCs to play around on.
Like, Minecraft is a true sandbox. There is no goal outside of what you feel like doing. There is a theoretical progression path, but nothing requires you to even consider it, e.g. I've never actually been to The End myself, despite playing Minecraft for years. I don't think such a thing could ever happen in a D&D "sandbox". Eventually you're gonna go face one of those epic challenges. It's just a matter of when, and which. That falls somewhat short of a true sandbox.
And part of why I say this is, I was one of the people in the first sandbox group. It was a lot of fun, and I look forward to playing more of it someday.