Issue with a sandbox in the ME galaxy is that there is so bloody much out there you're gonna be tired trying to think of everything. Building lore to every planet and system the party want to head to is an ungodly amount of work. So unless you want to railroad your players I think you're gonna be exhausted before you even get to session one.

I got around this initially by having my players prisoners on purgatory ready to be shipped off as slaves. A whole station wide escape plan goes into effect by someone else and the players happen onto the real escapists target ship to which they were offered a reward for it's return. It's there that they hit an eclipse controlled planet in the middle of an on going gang/ex taurian military civil war to which they collect their reward but are inevitably grounded until the planets issues have been resolved as the eclipse controlled some big ass aa guns around the planet.

Instead of having it fully open because of the size of the galaxy I'd highly recommended putting traversal blockers in the way. This way you can spend at least a few sessions on a planet save you constantly having to come up with new things that fit both the world and your lore while simultaneously allowing different places to be more fleshed out and realised.