I know this is probably not helpful, but I will mention it:
I mix my feed too.
What I have tried to reduce the mixing workload is:
Mix the high protein component of the recipe separately in its own bin (all seeds). Then all you have to do is take ____ scoops of the high protein component (all seeds premixed and concentrated) and add to bucket of low protein component grain that you measure out. (4 lb of oats, 10 lb wheat, etc.)
The advantage of this is that if you are only providing a few grains, you don't have to mix the GRAINS ahead of time (it is a lot of work to get all those seeds dispersed through the vast quantities of grains evenly and ahead of time).
This proves to be easy to mix, since you can just loosely take your hand through the bucket and quickly mix it
without having to worry about missing certain seeds. I am assuming that your feeders allow them to bill through quite a bit of the feed to select which grains and seeds they want to eat (free choice). This wouldn't work well if you are only offering a small amount of feed for them to choose from.
Sometimes during VERY cold weather I do a rotation of one day wheat/another grain, next day oats/millet/etc., then rotate through all my grains so that they receive ALL the grains over a period of about 3 days. This is mixed with my premixed mix of seeds/grains. I know this is less accurate and my protein % won't be on target. I also throw in a lot of sunflower seeds too sometimes as a guarantee of enough protein. This is my "I'm in a hurry mode." I have only gone into that mode a couple of times (so about 100 lb of feed).
I do this based on the human research that shows that you don't have to have the two parts of the complete protein all in the same meal or even day....I hope I am remembering this correctly.
The other helpful thing I have tried it to use a large oval galvanized metal tub and mix the feed with my hands. The downside of this is that then it is heavy when you want to pour it.
I just wanted to mention this in case you had not thought of NOT mixing your grains ahead of time- just do it by hand in the bucket before adding the seed mix.