When you got married and had your daughter you I'm sure thought that relationship was for life and that your daughter would grow up with her mum & dad together. She isnt and your marriage wasnt.
So you know any family blended or not can break down.
Your children are still very young and your new relationship not very long. So no I dont think it is a good idea for either child to call you mum and dad.
They have already gone through the separation of one family unit of mum and dad.
My dd has had a SM and step sister for 15 years. So she was much younger than your SD when they started their relationship. She has had a SD my DH and SB my DH son for 10 years. We also have a joint child.
Our respective children call their parents mum and dad and their step parents their names.
My dd loves my dh like a dad but she would feel very uncomfortable calling him dad as an older teenager. She probably wouldnt have so much when she was younger.
What works as a 5 year old watching a resident child interact with you might not work and cause issues when she is older.
You don't respect her mother and your belief that your partner are better parents is clear.
Your belief in the value of and strength of this relationship is also clear.
That doesnt mean it will last and if it doesnt it isnt the same as the breakdown of your marriage. A 6 year old could end up not ever seeing her second mummy ever again. That is not in her best interests.
Come up with a cute name if you feel you have to. Although I think the resistance to just being your name probably comes from your belief that her calling you mummy or similar will further cement your new family. It wont. Successfully blending a family and having happy and secure relationships takes more than a name and takes much longer than a year.