They love worms and all bugs.
Oats and worms would work fine, I think.
Pelletized feed does it for me, as I can be sure the pellets contain everything they need.
The 5 birds in the picture eat 20 kg of organic feed per year (never really checked tbh) @ 30€ cost.
But yeah: That is mainly soy and that is not environmentally friendly, I think.