I think you’re using your stepmother as a scapegoat for your father’s behaviour and have made her the bad guy in your story.
First, you say you get on really well, then add you have resentment towards her and took “subtle” shots at her. Said resentment is about vacations that you admitted you weren’t that bothered about when your half-siblings were babies and toddlers ( not surprised considering your age at the time) but then are annoyed that they don’t pay for you when you’re an adult, you knowing they didn’t really have the money.
You need to sit down and talk to them both about your feelings on the past so you can move through it.
You also need to actually think about this from a parent’s point of view since you hope to be one and consider what it’s like to be a step parent. Your father and stepmother had two small children while you were a teen. Most of the things you would want to do on holiday would have been difficult with small children. Your father could have tried to do a small trip with just the two of you, but who knows how much time he had to do so with your siblings, his job, your own schedule and whatever your mother and her side had planned while also considering the schedule of his wife. Did your mum ever take your siblings on holiday with you both? Would you have expected her to? Also, don’t forget that before your parents split they made many memories with you through the years. Didn’t your siblings deserve the same with their dad and stepmom?
You were harsh with your stepmother and even colder when you said you didn’t care. Your stepmother is allowed to be hurt and your father to be hurt as well by your words. They share a house and have done so for over a decade. He is not going to make his wife leave their home for the person who has purposely hurt her. I also doubt your siblings were impressed about you hurting their mum either so probably might also be why you’re not allowed into their home.
He is your Dad, BUT he is also the Dad to your siblings and the husband to your stepmum. He is not loyal to just you and hasn’t been for years so you need to acknowledge and accept that. At least, he is still speaking to you and open to meeting up with you elsewhere.
You’re going to find yourself eating a big piece of humble pie once you become a parent.