Barred Rocks are probably better layers than Cuckoo Marans, so your sexlinks might lay better if they have Barred Rock mothers instead. I don't know whether darker eggs (Marans-mix) or more eggs (probably from Rock-mix) will be better for your purposes.
Silver Laced Wyandottes could be the...
Yes, that plan would probably work pretty well.
Did you want to do sexlinks in several egg colors, or just one? I was thinking you wanted sexlinks with several different egg colors, so I was trying to think of sets of 3 breeds that could make sexlinks or autosexing chicks in all directions.
White Leghorn will not work for producing sexlinks.
Can you get a Brown Leghorn?
Brown Leghorn rooster x Cuckoo Marans hen will produce one kind of sexlinks (daughters black, sons black with white barring.) Those chicks will lay brown eggs, but a lighter brown than what the Cuckoo Marans lay...