In general, supplements are for when your certified avian vet feels there is a need for them, as like with anything , too much of a good thing is bad. Once a week is likely not bad. Everyday might be. The thing with supplements is you are never sure about exactly how much they are actually ingesting.
Keep working on the veggies, which are the best thing they could eat, and try all sorts. Or go with a 'chop' type offering. I use roughly 15 different ingredients, put most thru a food processer, bag it in 3 day amounts in baggies and freeze them. Thaw a bag in the fridge and mix with ground up pellets or nutraberries to absorb the moisture. Took a long time before my 'zon recognized it as food but he loves it now.