I started several threads re this last year,I just knew before opening this would be a dd.You wouldn't believe how many similar threads and posts you read re 6 year old girls.
I seriously think something happens with girls around 6,god knows what though.My dd was a little devil from about 5-6,a total angel at school and elsewhere but the hysterics over itchy tights,hair plaited,eating breakfast,the noise I make when chewing,my breathing when hearing her read
,face like thunder when I picked her up from school,stropping to school,posting notes under the door saying "I have the meanest parooons in the weld".... god it went on and on.
Like you I did everything,shouting,step,bedroom,charts.You name it we did it.Then during the last year we've had a complete change and she's lovely.Doesn't fly off the handle,excepts consequences,doesn't shout,says sorry,is a good girl most of the time and we have lovely quality time.She's thriving at school,totally loving it.My dd is bright(quite a perfectionist in a funny way) and she's madly reading all the time,writing etc.I think she's found her groove and is stimulated at school and knows how to amuse herself now.
I think something happens in year 1. They're not babies anymore and girls can be catty at school.I think dd was starting to notice peer pressure and was feeling vulnerable and tired. There is supposed to be some gland that kicks in at 6-8 too.In year 2 they're the oldest in keystage 2 and develop confidence imvho.
I'd just keep saying to yourself this will pass,don't shout as girls copy mums behaviour.I notice dd was distinctly better when I dealt with her calmly.I wouldn't threaten smacking as it's something she will do.
Try and ignore the behaviour by calming saying if you carry on you're going to your bedroom then frogmarch without making eye contact.Ignore the hullaballoo,threats and boasting coming from her room.It's all for attention and for the drama effect.
Try and have some quality time with her alone-you in particular.I found this helped massively.
Don't laugh about her bahaviour or make her character into a naughty girl as she'll play up to it.I was exactly the same at 6 and my parents regret doing that,my sister was perfect.My mum has laughed when dd has been naughty and referred back to me as a child.I've forbidden her from so much as smirking when she's naughty and told her I expect zero tolerance at all times.
Good luck,it's hideous but it will pass.Boys are waaaay easier.Soooooo dreading the teenage years.Dp has said he'll move out if I have the menopause at the same time!!!!!