You can do either. I think it’s probably OK to take the last chick out & take down the box as long as it’s about as feathered as the others were when they came out. That is the safest option because mom might get aggressive with it, wanting it out of the nest so she can sit on new eggs. And I think I mentioned before that some parents will stop feeding the chicks to take care of a new clutch, and let the chicks starve if they don’t figure out how to self feed. Fortunately parakeets do tend to wean quickly. The breeders who are all about volume, to sell to pet shops, will wait til most of the chicks have left the nest, catch all of them and then pull any stragglers out of the nests and transport them directly to the pet shop. So the poor things go from nest to having to eat or starve and luckily most watch the other chicks and start to eat pretty quickly. It’s a sad practice, but there are also plenty of responsible breeders out there who let the parents wean the chicks and make sure they are independent before selling them.
Thanks for the update,