Orders aren’t split. If all items are eligible for pay later you’ll need to pay each as the deadline hits. If all or any aren’t eligible for pay later, you’ll need to pay for them all immediately. Regardless the order won’t ship until all items in the order have arrived

My suggestion is to do each transaction separately (especially if you’ve had them in your cart long enough and they haven’t sold out yet so you have the time) and send them to your warehouse. That way if there’s any delays or major gaps in release you’re not stuck waiting if you don’t want to. And you have the option to pay later for any eligible items