I would just keep it as it is, changes are just unnecessary stress. You might want to add hutch space, like a second one and connect them with a tunnel so you can keep the buck seperated to give her privacy when she gives birth. Or to get out of her sight if she gets grumpy. I would do that, or divide the existing hutch if it is big enough as soon as possible so they can get used to it.
I would not seperate them, I put in a divider when one doe is about to kindle but they still get garden time together, and I don't do that longer than 1-2 days. The bond of yours might be less at risk to hierachy struggles (pregnant and nursing does don't engage in that much anyway), can't tell much about neutered bucks but my intact ones were always very friendly to kits. Keep an eye on him anyway, but I woudln't expect much trouble.
I would not split them up if there'S enough space to avoid it.
The most critical thing to kit survival imo is nestbox design and placement and making sure none is born or dragged outside of it, you can also start to collect fur from brushing etc, it's always good to have a stash of that if doesn't pull much or when it gets soiled.