She also sounds like a clone of Georgia
When she hit 6/7 mths of age she sadly turned into a bit of a demon towards our other 3 dogs....it eventually transpired that 95% of her behaviour was related to her seasons.
Her 1st season was at 8 mths & for 3 wks before, during & another 3 wks post season she would randomly attack any of the others, guard food or anything that dropped on the floor etc....we always had to make sure she was the last on the sofa & if any of the others jumped down she would often follow & 'tell them off' iyswim. She was also very jumpy/touchy.
After this it would just be very occasional behaviour until the time came for her 2nd season, which was just as bad.
I can't fully explain how awful this whole period was for everyone here & i really don't think anyone relaxed for the best part of 18 mths.
I had no choice but to have Georgia spayed after her 2nd season. This was now over 6 mths ago & the change in her has been huge.
Very slowly she is developing into such a lovely dog to live with
No attacks on the others
which is obviously the biggest relief for all of us
She will always be a bit bolshie, thats part of her character, but to see the change in her now is amazing.
I am so, so happy that we all got through this awful time.
I really hope you can find some solution for Darcie's behaviour. It
might be that having her spayed helps her too.