If you only got to see your husband for a few hours and couple of days each week and every other weekend would you think of yourself as having been awarded first place status? I would imagine that his children would gladly change time slots with you. Why can't you use the weekend evenings when the kids are there to schedule your alone adult activities?
You would never dream of saying to his boss," I am sorry but your work schedule is cutting in to my alone time with my partner. You need to change his schedule so that I get the alone time that I desire. However, in essence you are willing to send this message to his children rather than his boss.
Your issue is not his children's schedule but the difference in the way that the two of you prioritize his time. If asked, he probably thinks that he spends far more time in your company than he does in the company of his children. If there are activities that you feel that you are missing out on, why not ask him when can you do those things as opposed to suggesting that he give up time with his childr
Let him figure out when you can do a few of the things that you feel as though you are missing out on doing. He may feel that his need for alone time with you is being adequately met.
In any case, it seems that your real problems are neither a time or step child problem but rather an inability to communicate and a difference in perspectives.
You might consider couples counseling to help each of you hear and see from the other's perspective. You might just be happier in a relationship where you are the only person with whom your partner has to consider when deciding how to spend his time.
It is often a mistake to make assumptions when you have the ability to clearly articulate your expectations.
Why can't you