Couple of suggestions:
First, you said you used 2,4-D. I would suggest a general use herbicide like glyphosate instead, as 2,4-D is specifically a broadleaf killer. It will not kill grass. I don't know how long you waited between the spray and planting, but 2,4-D can have a residual effect on some broad leafs, and could have possibly hurt your germination and growth percentages.
Another thing I would suggest. Get 4 cattle panels at a farm store and wire them together to create a small exclusion spot. You will never see some of the things the deer eat, nor know how much they eat. With an exclusion spot, you have a benchmark. You can see how much the deer are utilizing the plot, and you will also be able to see if germination was a problem or if the deer eating the emerging seedlings played a role. And if you have lots of rabbits, put something at the bottom to exclude them, too. Then you won't accidentally be misattributing the lack of growth or density to operational failures and trying to make changes IF the problem was wildlife use.
Sunflowers are probably something I would skip in most food plots. They aren't consistent performers and there are much better forage options for deer out there. If birds are a plot consideration, there are still better options, like grain sorghum.