The key to keeping a sandbox running is your ability to index and rapidly reference the volume of information that you and your players will generate session after session. Having strong organizational skills will be a boon, but just being aggressive with whatever data management solution you choose will work. I have spreadsheet pages filled with notes from my year plus Edge game and that one is only semi-sandbox.

In my experience, most players don't want a pure sandbox. Having a wide-open universe causes their brains to fry with indecision.

So what I do is a game that appears to be a sandbox but hidden underneath that sand are rails. It's all about "the illusion of choice."

In OP's situation I'd probably come up with maybe 3 to 5 basic story ideas and gently nudge them towards one depending on their actions and choices.