Need advice on rats who bite calmly but HARD!

Hello! I've been quite active in this subreddit recently, and I'm back hoping to get more advice. One of my previous posts has more details on the situation with me and my four girls, but the important part is that I've recently gotten rats with some behavioral issues, and they're very prone to biting. They're extremely skittish and only one will come up to me without treats, and all of them are very easy to spook and don't enjoy handling. My brave little girl is the one I've been working with the most since she's the only one who comes up to me consistently, but I'm prioritizing getting all of them comfortable with me and I try to focus on the others when they're willing to come up to me. I've had them for about 4 days at this point, and I want to make it clear don't expect any change in their behavior yet! I'm willing to take my time with these girls and move at their pace as they grow to trust me.

The biting isn't just the issue, it's the way they bite so calmly without warning. I've struggled to find people with similar situations to these girls, as they are NOT aggressive and not biting out of fear, and I'm sure my hands don't smell like food. They will come up to my hand, sniff it calmly, maybe give it a few exploratory bites, and often walk away with no issue, but occasionally they will bite HARD. They don't hold on but it's a decent bite, one of my girls got me today and I can see some fat poking out of the bite wound. They show no signs of fear, no warning signs, they just seem really curious and interested in my hand. My one brave girl is the worst since she really wants to come up and interact with me, but I can't trust her not to bite. I've been providing plenty of treats, and I sometimes offer my hand to them between treats, with my hand balled up in a fist and held by the edge of their open cage. I only give a treat if they don't bite, I walk away if they do. I also squeak at them if they do bite. I pull away my hand and squeak anytime they open their mouth to start to bite. I'm concerned about correcting exploratory biting since it's a natural behavior and how they understand the world around them, I try to let them bite gently to explore my hand but I've had gentle exploratory bites turn into very nasty ones with no warning, so I can't trust them anytime they open their mouth.

I'd really like to focus on fixing the biting since I feel like if I can't trust them, it will be even more difficult for them to trust me. I was hoping to get some advice on this sooner rather than later so I can use the best training methods and avoid reinforcing bad behavior, since I'm struggling to find resources or people who've experienced similar biting to this and I feel like I'm kind of making up this training as I go. Any advice would be appreciated!!!!