Google cockatiel/parrot training and look up 'parrotwizard' on youtube. His stuff is great.
To make this brief, stop shoving your hand in the cage. Show them treats like millet, they fucking love that shit. Make them reach for it. Only put your hand in the cage when you are changing out food, water, and to offer treats. If they freak out, back off. If they calm down, leave your hand in the cage. Don't try to grab or make them perch. You need to reach them your hand is safe and they won't trust you when you are always grabbing them.
Some birds never get over cage aggression, especially if they have lived their while life stuck in a cage. I personally prefer to train outside the cage but you need to bird proof your room if their wings aren't clipped. A scared bird can easily kill itself by flying into shit.
Entice them outside the cage. I tricked mine out of the cage and closed it behind them. They had a perch on top of their cage where I let them hang out. I also have a flight room where there are some perches for them but they didn't like to be so high or two low so they ended up just flying to me. Training clipped birds is easier to me but Ive only clipped their wings if they were a danger to themselves or came to me clipped. A clipped bird is forced to rely on you but restraining them to clip wings is pretty traumatic to them.
In or outside the cage, keep them close to you but not too close. Go about your day. Talk and whistle to them softly, offer treats.
But really look at parrotwizard's stuff. It's great.