I've been a step dad twice in two separate relationships and my daughters had two separate step mums.
I've expereiced difficulties with all those relationships in two areas- 1/ jealousy from the child towards me or jealousy from me wanting more time with my partner 2/ not being on the same page in terms of discipline.
Thankfully you have the latter sorted- believe me, that is a gift.
In my first step parent role (30 years ago now) the boy was 2 years old and his possessiveness of his mum was unbearable...but understandable. We only had a relationship after he was sound asleep and even then 2 hours later he'd wake up and want to get between us with no help from his mother to get him back to bed etc.
The second time was with a 14yo boy that was more full of conflict and rejected most of my kind advances towards him.
Then another partner had my daughters as step children and I felt the reversal of all that drama. So both sides of the step parent world is hurtful and demanding.
It was so bad that once I split with the last lady I vowed I'd not have a defacto relationship again. Yet, my best friend a lady with no children and indeed close to my daughters, I'd end up marrying and she is "mum" to my eldest so I got it right but there is no complex relationships to sort out.
I suppose my main message is that I believe step parent situations rarely work out in harmony. A lot depends on the parent, step parent and childs personality and how much contact and harmony is present with the natural parent. For this reason Romantic Thiefs recommendation to get your own psychologist is a master stroke because you'd be amazed at the number of suggestions others can make that can improve the situation.
Eg. My 14yo step son when angry would throw things around. I took the advice that it was his action not mine so not to worry about preventing the problem. One day my step son was angry at me and threw his jumper onto the roof of our then two story house. I ignored him. When his mother arrived she wanted an explaination from me. I suggested she ask her son for one because I didnt throw the jumper on the roof. She wanted to pin me down on the subject somehow but couldnt. Eventually she asked me to retrieve it with a long ladder. I refused as it was high and dangerous. Days later my step son asked me very nicely to help get the jumper back. We worked as a team, him holding the ladder and securing it with ropes- I got it down.
A professional is good value