My own experience with F1 is that is best to requeen EVERY year with bees bred by a specialist who knows exactly what they are doing, and has the resources ( and a reputation of excellence to keep) to breed from good selected stock.
NOT something easily undertaken on a hobbyist level.
Professional breeders have hundreds of colonies, a dedicated and well trained staff, and an area that they can manage to isolate their breeder stock.
Hence the Danish and German F1 breeding islands in the Baltic.
I can not see how any hobbyist can get repeated quality by open mating... particularly when the beekeeper down the road keeps Greeks, another Carniolian, Cepolian, Causican, Heinz 57 etc etc.??????????????
Yeghes da