I think cage is fine for a cockatiel! , Veggies and fruits take time. I introduce my babies to fruit when I wean. Many breeders I know don't even give :-( . some will look at it not touch it for weeks, some will try it, some will only try some at first..... BUT Always fresh only. and keep offering.... lol harder for those that never had it offered. They will though. and through that they will pick their fav's also.
With fruit, please be aware that cockatiels are notorious for not liking fruit! If your little one rejects it and never touches fruit, don't be worried, it's a known 'tiel thing.
With veggies, cockatiels can love them! Mine like carrots, broccoli and spinach, and they love their sprouts too. Gentle, persistent offering is the key, and maybe showing yourself eating some (but do NOT share the bits you've eaten, keep your germs to yourself!) might also encourage them. It took a while for my Older Birb when younger to get into them but she does like them. Younger Birb LOVES LOVES LOVES her sprouts and has a good go at her other veggies. Neither like fruit; Younger Birb DID try a blueberry but discarded them, and decided that raspberries were outright scary and hissed at them (which was hysterical).