It does sound complicated to include potential step-children in the trust.
However, I assume that you are not putting every penny you have into the trust, and that you will also have access to some other considerable savings? If so would you be in a position to make an ad-hoc decision if a step-child situation arises? Whereby, for example, one step child joining the family at age 8 might be offered [paid by you from your savings] the opportunity to also attend private school. Or at the time, in the event of there being step-children, negotiate with your dc how a car is funded between you for a teen / young adult step-child.
There would be no legal commitment but still the opportunity to make day-to-day opportunities feel equal for those in a blended family.
If your dc were in a relationship with someone who already had multiple children, it would be part of the negotiation of their relationship that a biological child would have opportunities not affordable to all the step-children.
I think funding school fees should be considered since it is about equality of education and opportunity. Funding a gap year or paying university fees is not always necessary or required, and can be funded in other ways.
Beyond providing for your own dgc it seems way over complicated to try imagining scenarios of who your dgc might be in a relationship with or how that might play out. I think you have to let go of that level of forward thinking. Imagine a trust fund set up by your grandparents... probably nothing for any same-sex marriage (it didn't exist), probably nothing for your own children born before you were married.
You can only do your best now, and as situations arise look at what you can do at the time to maintain family harmony.