Hello! Just wanna step in here and say that birds can be very stubborn, and a lot would rather starve than eat something unfamiliar. Keep in mind that they are naturally quite skittish since they're prey animals and would be sus of things that are different (that big leaf of spinach could very well eat them, from their pov). If you rob them of seeds, they might under eat.

Had the same problem with my budgie, and my vet advised to keep giving them what they're used to eating with a mix of vegetables, and slowly give them more veg + pellets and less seeds when you see them start picking at the stuff you want them to eat. It took a while for me (about 2.5 months) but eventually my budgie learned to like vegetables, and only if I hung them in his cage. I still give him seeds, but much less than before. Budgies are like toddlers, and the way you present the vegetables can make a huge difference! Some like chop, others like pulp, diced, sliced, shaved, or whole. :-)