A belated addition to the post. I also buy in bulk direct from the grower/growers as much as possible. My seed is stored in 240 litre wheelie bins which hold close to 200kg each.
I feed my seed separately in multi-bay feed hoppers. Finding a suitable mix would be difficult. I find in the warmer months that Red Pannicum is the dominant seed taken but the weather changes they switch to Plain Canary and far more White Millet. At the moment my birds are taking very little Red Pannicum.