Of course we an on taking things slow
You may feel everything is moving slowly but eighteen months from leaving one marriage to engaging yourself for another is not what I would consider slow.
What stage is his divorce at? Have they managed to agree a financial settlement ?
How many of his other friends and family have you met yet?
What are you thinking, considering having a baby with someone you really don't know very well, with whom you have never actually lived and whose current marriage is so very much not already ended in the eyes of the law.
Now look at it from his DD's point of view. You know you are a lovely person and want to make a home for her. She only knows that her DF seems to have moved on.
As far as his DD knows, only eighteen months ago he left her mother. March 2020 was a strange time for many of us and some things are only very slowly getting back to some semblance of normality.
Now she knows that he has a new girlfriend, which is a new level of finality for her parent's relationship.
Can you imaging yourself into her shoes. Would you want to meet you, if you were her?
What what you have told us, there is nothing I see which suggests a sensible way forward or any happy outcome.
My strongest hope for you is that you are using very reliable methods of contraception. And that the DD does not have a baby at the same time as her DF's girlfriend.