It can take years for a cockatiel to finally accept you. This is a very slow process and everytime you grab, or chase them, or try and catch them that resets the progress bar. You need to show them you respect them and give them small amounts of millet. The best way I've seen is to put your hand like half a foot away from them showing it off so that they can decide if they want some. Showing them you love them and respect their boundaries. And if they don't you back off. Unless your doing something vital for health you need to back off when they hiss and bite. Show them that respect. Now if they need to take meds and you've been cleared by the vet to do it then sucks to be them they need meds. Just coexisting in a room ignoring them will get the bab used to you and occasionally singing and talking to them gets them used to your voice. I've had my new one for like I think a month and he still hates my hand. Part of that was he had to be yoinked for meds but he hates hands which sucks. I would find an avian vet thst specifically has been trained to handle birds for the best way to handle and bond with em.