Yeah, depending on where you live, it can be *really* hard to find good food. I can only find two kinds of pellets locally and one of them isn't much chop. Thus, I have only the one choice (Vetafarm Nutriblend). I buy my pellets and backyard mix (it's chicken feed) from a feed merchant near home.
It's also not that easy to find a vet who's interested in birds (unless you're near a large city where such vets might occur). I'd bet most of the ones in your area would be large animal specialists? You could try a stock and station agent or a feed merchant and buy a variety of seeds to make your own mix? I have occasionally paid my kids to sit and pick the sunflower seeds out of Trill wild bird mix. That presupposes you have access to bankrupt kids, though.
Bottlebrush (Callistemon species) and Melaleuca have small bottlebrushlike flowers, rough or papery bark and are usually covered in bees. The fruits are clusters of woody 'nuts' arranged in lines along the twiggy stem (birds LOVE these). The foliage and fruit of any native tree is suitable and you know you're safe if you see native birds munching on them.
Hand on heart, my personal opinion (which is nothing in the grand scheme of things and you must make your own decision) is that you should keep Sunny. The fact is that he's no longer a wild bird, but a domesticated one. He finds his food and water in his cage dishes and his society in you. Without a dedicated pair of parents to teach him, he'll always be 'behind' in terms of a wild flock.
If he had been a successful adult who was injured and you had rehabilitated him, it'd be a bit different because he would already have known how to be real corella. Assuming he's a nestling, only fledged in this year's crop of chicks, he's lost his chance at imprinting on successful parents and learning the ways of corelladom from them.
I think that even if you released him with wild corellas known to him, he would fail to keep up and fail to thrive in the wild. I reckon he'd ultimately fall victim to a hawk or falcon because he probably doesn't know to avoid them. Or, he'd fly down to someone's yard, begging for food and probably be re-caught and caged by someone else.
It's your decision to make, though. You know Sunny better than anyone and are in the best position to know whether he's capable of sustaining himself away from you. Keep encouraging him to fly often because he'll need a lot of stamina if you do decide to let him go. Also, give him lots of practice at flying UP and then flying down again so he can practice taking off and landing from heights, from the ground and from unstable perches. He needs to learn how to fly in a circle and to duck between branches and through confined spaces. He must learn where the grass seeds are and that water occurs in the ground, not in a ceramic dish. All this can be taught, but it's a difficult and constant job. I rehabilitated a magpie once (geez! that was about fifty years ago!) and it took forever to teach him to fly.
Main thing is, don't stress. Just take care of Sunny from day to day and he'll let you know what to do for the best. While you're working that out, enjoy each other!