I think step-parents need to remember that they are very much allowed to have needs and wants. Biological parents in all family set-ups do, step parents often get sidelined, and it seems to be to a far greater extent when they are a NRP-step.
I feel for you OP just because your partner is a NRP doesn't give him a pass on caring about your needs or wants.
i think the not living together is a bit of red herring, as you say that you see his DC a lot and therefore it sounds like your mostly living together? Certainly when with them?
The OP wants to celebrate her 30th birthday, she has shared this fact with her partner, including when she wants to celebrate he has decided to take his children away, she has been told that's what is happening. Change children for anything else and he is being very unreasonable.
There are 52 weeks in a year, he doesn't have to take his children that particularly week. His children don't need to go on holiday that week, he has made the choice to take them that week and as a consequence the OP doesn't get what she wants for her birthday, or what she needs to feel loved, appreciated and a part of a wider family. It's a special birthday, it is important to her, it should matter to him because it is important to her.
It may cost a bit more at some other point in the year but it is his partners 30th birthday, she deserves some consideration she on this occasion IMO deserves to come first.
All I can see is that when it is convinent for him, the OP is very much a wanted part of a blended family. When she wants or needs something she is just told no and the children are used as the reason. This will build resentment as the OP feels less and less important.
I'm a RP-step but in our family it is accepted that everyone has needs and wants, wants are never put ahead of needs no mater how old you are. DC needs are always put before us both, because that's what adults do.
Going away alone with his DC is lovely, but it shouldn't be at the cost of the step-parent or the wider family unit. If this man lived with the OP and i am guessing her children, what message does that send them? It has to be equal for everyone.