I know what you mean about the suet mixture getting very hard. I'm new to this and have only made two suet batches. Could we be getting too much lard in the mixture and that makes it harder?
I have tried three types of seed here in East Tennessee: mixed wild bird seed, regular white striped sunflower seed, and black oil sunflower seed. I noticed there was a lot of waste with the mixed seed-the birds would kick out the small seed. The birds ate the striped sunflower seed fairly well but it did not attract a wide variety of birds. However, the black oil sunflower seed became the main attraction so I fed that, exclusively.
I noticed in a small bird book the various types of seed they recommended so I decided to experiment. There is a local feed mill here that stocks a wide variety of seeds for their own blend of feed products for livestock. I bought five pounds each of safflour seed, wheat, millet, cracked corn, and milo. The owner explained to me the reason why the birds were kicking out the small millet seed in the store bought mixed seed was because it contained the wrong kind of millet. The type used in that commercial variety is very bitter. He said they would love the kind of millet that he had, and you know what, he was right!
I have two sections of metal channel from a four foot long florescent light (they are about 4 inches wide and a half-inch deep - they were the protective coverings for the ballasts). I placed both four foot sections on my deck railing and grouped the five different seeds together with black oil sunflower to see what happened. Ha, an eight foot long bird feeder! The results really surprised me:
Many birds now only sparingly went to the black oil seed. The safflour seed is a favorite for cardinals, they love it. Black-capped chickadees, tufted titmice, and house sparrows like it also. A variety of birds like the small millet seed with some eating the milo. Wheat does not last long either as doves and many other birds readily eat it. What really surprised me was the attraction of blue jays to the cracked corn. I had no blue jays until the cracked corn was made available. It is not unusual to see four at one time now feeding on it - they love it! I have observed titmice, chickadees, and cardinals eating cracked corn as well. I think if you could get these varieties of seed you will be surprised at what you might see at your house.
I have made a three foot long hopper-style birdfeeder with five compartments. This is to hold the different types of seed used from my little experiment. It will be a lot of fun noticing the attraction that the different seed varieties provides.