My situation is a bit different, because my DSCs live with us all the time (their mum is deceased). So there is not that dimension of 'limited time' with dad, IYSWIM. DH and I have a DD together, as well.
I agree with you that being a part of the family is really, really important, and I don't think that 'solo' time with dad should be at the expense of that, or instead of that.
On the other hand, I think in all families, getting one-on-one time with parent(s) is a great thing. I grew up in a large 'intact' family, and I still craved (and did sometimes get) time with my father, just us, and with my mother, just us. It's not just step-children that need solo time.
I try to make sure that DH spends time with each one of the three kids one-on-one. Sometimes that's easier than other times! And a lot depends on the kids' ages and temperaments. Because I have one DSC who is more demanding and has in the past really dominated things, we have to make more of an effort to 'even things out' and find opportunities for the other two children to have his attention.
In our home now, there are in some ways, four family units. There is the unit that includes all of us - DH, me, DD, DSCs. There is the unit which is me and DD. There is me, DH, and DD. And there is DH and the DSCs. I actually think this is very healthy, and it is definitely the way that works best for us and our particular situation. Sometimes DH and the DSCs do things separately. Sometimes DD and I do. And lots of times, we do it all together.