If only there was a solution to this conundrum.
I recently bought a bag of Tropimix, it´s a 100% edible seed, nut, veggie, pellet mix using human logic of ¨well, my birds like the Tropican pellets, so they will probably like this mix¨, how wrong I was. Most of it ended up outside for the sparrows. Yes I wasted my money, but it´s a learning curve. Now I know they won´t eat it and I won´t repurchase. At least the sparrows and doves got a feast...
The best thing is sample bags, if you can get them, do it. Tka says there´s a sample bag 50g for a pound, fantastic! Defo get one and give it a go before buying a large bag
If you have friends of family nearby that own birds, you can share food (ask for half a cup of food to try it out).
Good look with the diet conversion, and the veggies. Be consistent with veg, sometimes they will surprise you and eat something one day.
As for the vitamins/ supplements, one of my girls had a bit of bother recently and we were given a supplement in liquid form to be given orally. I would recommend having an oral supplement because then you know they got the dose. No worrying about are they eating the powder, drinking the vitamin water.
You can ask your vet for a supplement, they might sell bottles or have a good recommendation. There are lots of ones available.
When the chicks hatch, you can prep some formula and feed it to the parent that feeds the chicks once a day. This way the little ones get the benefits from the formula, and also enjoy being fed by their parents. Which formula do you have?
You can even offer soaked pellets while there are chicks in the nest. Just remember they spoil fairly quickly, so don´t leave them there all day.