Consider how you would feel for your little one, if in 8 years time her daddy left, met another woman and then moved 6 hours away to be with the other woman and a new baby.
I’m sorry, but your career satisfaction is far less important than your partner’s children and even asking him to move away from them is spiteful.
I assume he had these children when you met. Did you think at that point it should be so easy to just up and leave so that you can pursue a career?
I understand that it’s disappointing and perhaps you partner did ‘imply’ at times a move ‘might’ be considered, but you chose this life, you.
In my opinion your partner is right to stay close to his children. Even a 1.5 hour move is too far, what if he was needed in an emergency?
As you now know that your desired career is out of your reach at the moment, work towards something else. I trained to become a product designer and at the time, those types of jobs were in and around London. So I did a PGCE in design technology and became a teacher instead. I’m on £42000 after being in that career for a while, not a huge amount, but certainly better than it was and you get the holidays with your children. If your husband retrains too, he could bring more to the household and again raise your overall living standards - a move 6 hours away is not the only option.