Hi Blarneyeggs nice to hear from you.
I m sorry you had to get ready of your best White cockerels but it s a nice move no deed agressive rooster.
So maturing a a trait passe down from parent. your line is slow maturing that all. It s not related to variety ,it s related to line breeding.
To fix that cross them with your Black copper ( if you are hapy with your BCM) = you will get F1 Black copper ( Split White C/c+ ) than Cross your best F1 rooster to his sisters/half sisters = you will get the F2 and 25% of them will be White New blood line.Select from them the early maturing and the dark egg layers.
I think your plan to keep them with a cuckoo in the same pen is good and will work because the silver cuckoo are silver bassed chooks very good gene to keep the white feathers pure.
BRAVO for your BCM pullet Best of the breed and reserve of breed.
Breed here with your white cockerel that will work.what egg does she lay?
That exactly what i done last year I crossed my white 1) Birchen maran ( I m expecting a good type from this cross)
2) Silver cuckoo maran.( I m expecting a pure white plumage .pure white chooks and dark eggs)
3) Wheaten marans ( type and dark egg laying )
4) between them selves ( to keep the line pure for a futur back crossing)
So this year I have 4 different crosses to work with Happy man.