I am out of ideas

My daughter will be 7 next month. Her biological father has never really been in her life and it caused Adjustment Disorder. We see her "feelings doctor" (pediatric psychiatrist) twice a month and have made great progress!

Her Dad came into her life when she was 4, they get along phenomenally.
Little sister was born in 2020, set us back a bit with her Adjustment Disorder but nothing too bad.
Baby sister was just born last month (it’s the last baby) and this time… it’s different.

Lately everything that Dad, or even I, say results in a full-fledged meltdown. She never had tantrums as a toddler so it’s relatively new to me to be seeing (especially at almost 7) her screaming and flailing.
I’ve tried talking to- not at- her about how we should behave, tried asking her to talk about her feelings, asked if she wants time to herself, asked if she wants one on one time with me or her Dad… nothing helps. It can be something as simple as asking her to brush her teeth before school or putting on shoes.

I’m worn thin and out of ideas… has anyone dealt with similar, and what helped you and your little get back on track? I miss my happy little girl