If she's five and displaying this behavior it will take a lot of time and work to bring her around. I'd also bet she was like this with her prior owner (not the rescue) and she learned that when she hissed and threatened she was left alone.
After giving her a few more weeks to settle in I would start to tame her down. Obviously make sure her heat/humidity are on point and she's well fed but not overfed. I wouldn't be surprised if she was underfed so she wouldn't outgrow the 20-gal tank. Much of her attitude may simply be hunger.
There are a few tricks you can use to teach her that you are not a threat, and that hissing at you doesn't make you go away. One is to put a shirt you've worn into her enclosure. She'll figure out that the smell of you in her home doesn't hurt her.
Another is to bring a chair along with a book or your phone into the room, sit down next to her enclosure, and spend an hour or so quietly reading next to her enclosure. Ignore any displays or striking. The goal here is to teach her that no matter how much she threatens, bluffs, strikes, etc. you are not going to react and you are not leaving. Try to only get up and walk away when she's calm.
For day-to-day maintenance you may need a hook to keep her head away from your hand while you spot clean or change water. One trick I've used with my retics is to rub the hook with a bit of lemon scent hand sanitizer and let it dry. It's a strong and sharp enough smell that when they encounter it they want nothing to do with it. I also use a bit on my hands, especially if the snake is stupidly foody, but even a defensive one backs off when it smells that.
She may never settle down 100% but if she gets to the point where you can maintain her without bleeding then consider that an accomplishment.
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