D.K.
You are doing the right things and I have a six year old we just got to the bottom of some of these behaviors! I feel your pain!
First, kids aren't bad because they want to.
Maybe make a visual chart for her, she needs to earn priviledges not have them handed to her, she is to do her work, bring home her stuff from school, not sass and so on and earn the right to watch TV and so on.
Then if there are good behaviors there are smiley faces that are earned. Acknowledge the good behavior as it happens. At the end of the day go to the chart with her and see what she can do the next day to make it better.
It helps some kids to see for themselves how their behavior is visually.
If she slams a door, very calmly ask her to open and close it four times NICELY as it should be. If she sasses, make her stand there and repeat what she said until she says it with respect and nicely. Acknowledge her without arguing with her. Pointless I have learned and exhausting to argue with a 6 year old! :)
Your daughter is way too old to be sleepig with you. She may seek that security from you that she needs to get during her waking hours. A girls day out once a week as a reward for sleeping in her own room. Maybe a new bedspread from her choice if she sleeps in her own bed. Putting her in control over how her day goes, how things go between the two of you is really good for them. She can either be in trouble or be rewarded for doing the right things.
She is testing you to the max right now and finding your weak spots to see where you will break. No matter how hard she tries don't give up.
You are very aware of her and her ways of doing things and have implemented discipline already so that is great.
She may too be asking for attention from you in the only way she knows how. Sit down and talk to her when it isn't a fight or arguement and ask her what she needs from you or what would be the perfect day for her.
I have taught (after a LONG struggle) to teaching my daughter it is OKAY to be upset, angry and not like my rules, however she is to be respectful, listen to me and if she is angry it is better she calms down then we can talk about it. My daughter seems to be analytical so explaining why I have the rules I do seems to help her, she needs things laid out for her more directly. I have had to teach her that her actions and words when she is upset are going to be her in more trouble then just her accepting she made a bad choice and has consequences.
Consequences for children is huge and I think so important. Teaching them to take responsibility for themselves.
If your daughter if feeling she has no power or control. then she will act out too. Maybe give her some responsibilty around the house. Help her choose meals, help you cook. Give her choices. It is tough being a single parent and even tougher when you have a child that is challenging you. She isn't deliberately being bad, she is seeking something and you need to figure out what it is. That is the hard job! :)
We went to a play therapist last summer as I was at the end of my rope and we got so much accomplished! I found a way to communicate with my daughter she found a voice to talk to me about her feelings instead of acting out on them. I learned not to yell so much, to listen more and to not try and control her so much as teach her to control herself.