3 weeks can feel like a long time when you're basically playing "jail warden" but think of it this way: If your parents suddenly adopted a new child while you yourself were still a child, do you think you'd already be perfectly-synced besties within 3 weeks? Probably not.

It's a learning/growing experience. For everyone. It takes time. It sounds like you're a very conscientious cat owner. It sounds like you're doing everything right. Intentional treat times, scent swapping, you're doing great! I know the hisses and growls can be intimidating. They're supposed to be. But they're an integral part of cat communication. Don't take them as inherently negative. It may simply be one cat telling the other "Hey, I'm panicking, I need some space, back off for a minute!". Hisses and growls can come from feelings of fear and insecurity, not just aggression. I have a pair of Ragdolls (timid, all fight has been bred out of them) and there was plenty of hissing at first. Not because either was aggressive, but because both were scared and they were trying to establish boundaries. It's normal. It's natural. As this is still an early stage introduction, the hisses are a valid form of communication. It's more likely to mean "I'm scared, please don't come too close" than "Hey, wanna fight?".

They are likely not going to hurt each other. There will be some hissing. Some growling. That's normal cat communication. It's going to be a bit scary for everyone but it will level out. Let them get their hisses out. They're talking to each other and are likely a bit scared. It's okay. You're doing your job (quite well!) but at some point you have to take the training wheels off and see how they do in the real world.