I would also remove the nestbox, and not breed these two particular birds.
That’s great that you’ve taken your time to research, and have not rushed into it. Keep in mind too, that not all “breeders” out there are up to date on their practices. Not sure about the ones you’ve asked your questions to. In fact, a good amount of budgie breeders do things the ‘old school’ way, and not necessarily the best way. If you haven’t already done so; by reading both the Stickies and the threads in the Breeding section here, you’ll pick up some more tips regarding best practices for breeding, as well as what we don’t recommend.
Enjoy the forum