While I don't have the number of birds you guys do, I have tried growing seeds from the seed mix and have found it surprisingly easy. Unfortunately the budgies haven't been as enthusiastic to try as I have been to grow, but have found leaving the pot unloved seemed to encourage the seedlings to keep growing once they were past the sprout stage. I have quite often found little millet heads on some but the only time I tried the budgies with one, I can only describe their reaction as "spat it out". After Kaz's comments, I think I will persevere, hopefully they will come around to my way of thinking.
On the subject of grasses, seeded or otherwise, I give mine a little bundle of grass each every morning, either from the back yard or pilfered on my morning walk. They love it. Not so keen on the leaves, it is more the stalks or the seed heads they go for. And yeah, any old grass. Even the lawn grasses like couch and buffalo.