You've got a great system going!
I don't breed, for multiple reasons. If I were to breed, cockatiels would not be one of those species. (I suspect bird keeping in Spain is drastically different than here in the USA?) With the exception of Casey, my entire flock consists of second hand, plus, rehomes. Most of the birds I've taken in have been unwanted birds, one was close to death (starvation because they "simply forgot" to feed her... aka the pet they found outside ended up not being the pet they expected her to be). Some of the birds I've taken in I've found new homes for, others I've kept for several years until failing health, potentially related to age, has taken them from me. Charlie and Jayde came from foster homes, Jayde actually from a member here. Sylphie from a local bird rescue.
I have been speaking a little with a local breeder and she introduced me to her flock. She handraises all of her birds and the first foods that she introduces to her chicks are sprouted seeds and vegetables. No dry seeds or pellets! They get that once they are older! I wonder if it might help switching the parents to a similar diet while they are breeding and raising chicks?
Some parents are just bad parents though and may over-feed, under-feed or even mutilate the chicks. Sometimes, it can help to put them next to an experienced couple during the breeding season. If that doesn't work, it might be a good idea to discourage the pair from raising chicks again unless you are willing to hand raise all future clutches.
I have heard of some adult birds that their parental instincts are so strong that they'll help to feed any hungry chicks, even if they themselves do not have a nest, nor a mate. This is typically seen in female birds, but males will do it, too! Other pairs wont accept offspring from other birds *unless* they have their own eggs/chicks.