Can you freeze this casserole, and if so, at what point?

Yes, you can make this ahead and freeze it before baking. You can also make it up 24 hours in advance and just leave it in the frig. Either way, leave the crumb topping off until it's time to bake, though, or it will get soggy and stay that way.

I've never met a casserole that doesn't freeze well...I would just make it fully before baking, wrap it well in plastic and then freeze it. Afterwards, you could bake it from frozen, just adding on time to compensate for the thawing.