I've been in your position. Best thing I did was speak to autistic adults to understand what autism is and how best to parent an autistic child and to get rid of any misinformed ideas I had about what autism is.
Google is not your friend. It's totally misinformed. And neither are professionals who use phrases such as "on the spectrum" and "red flags".
You need to speak to autistic adults ASAP to get it right. You need to learn and open your mind that a different neurotype is not the end of the world, in fact its something that you can celebrate, with the right approach.
The idea that your daughter won't be treated any different needs to change too because she is different, she requires adaptations in her life to make her world more pleasant and tolerable. You need to get a good grasp of her needs and ensure that they are adhered to at all times.
Autism Inclusivity is a great group on fb. Ran by autistic adults who answer questions from parents like us about what our children are going through and how best to help them.
As for your DP and her DD, it took my husband a couple of years to come round and a formal diagnosis to accept it. But you know what? I wish I had had the benefit of burying my head under the sand and just been able to enjoy my son in those early years/months, rather than having to fight for every single service and being anxious about the unknowns.
We have had couples therapy since to ensure we are firmly on the same page, for the sake of our son as much as anything, because he needs consistency.
I wish you all the luck. Sounds like you have a good start to the journey and your daughter will be lucky to have a mum like you. It won't take long for you to see that it's not all about red flags and doom and gloom xxx