How do I get my cockatiel to not hiss anymore?

Charlie always hisses at the fingers/hand and then bites. However, he's as far as hopping onto the hand when he sees millet and lets himself be lightly touched while eating millet(he does not seem to care to a certain point; he's usually like "nah don't do that, but I kinda like it and I'll let you touch me while you give me treats."). Our lutino tiel doesn't hiss nor bite, he's very gentle, but he's also not hopping onto the finger/hand without holding millet as a treat(it seems he's still scared at some point, Charlie the most though, he HATES the finger for some reason). Recently, Charlie also seem to show more aggressive behaviour than the first 2 weeks he's been with us. He's more aggressive to his brother and also more aggressive to the stick(which he usually hopped onto with no problem, just like Nico (our lutino), but even he suddenly refuses to hop onto the stick anymore; only if there's millet). They also sometimes shiver, but that could also be because we live in an attic apartment?

Any advice how we can teach our tiel to not hiss anymore and show him that the finger only brings good stuff and isn't going to hurt him? Kinda goes for our lutino too. I know taming takes time, but they seem to have gone one step forward and two steps backwards and we're lost on what to do ...

I'd appreciate some step by step advice from some experienced owners <3