You don't need to go severe with rationing seed and if they like the other foods they won't starve themselves- it's if you give them foods they've never had and don't give them another option that they can be fussy.
My budgies always have the option of seeds and veggies and they love to have their healthy greens as well as some seed.
Limit them to 1 1/2 teaspoon of seed per budgie and even if they are still having seed they won't have too much.
Sometimes when I feed my birds in the morning I use a chopper to dice up the veg into small seed-sized pieces, I add a bit of seed and sprinkle all over the freshly changed paper at the bottom on their cage. They love it.
My birds have separate dishes each with a different brand/flavor of pellets available to them 24/7.
Their seed is rationed to approximately 1 and 1/2 teaspoons of seed per budgie each day.
I give them seed first thing in the morning right after putting clean newspapers on the bottom grate of the cages.
I sprinkle their morning ration on the paper so they can forage for the seeds.
I then do the same thing in the evenings (after replacing the soiled newspaper with clean) and again allow them to forage for their seed.
All of my birds have the option to eat the available pellets whenever they like throughout the day and they all enjoy them.