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I have never written fiction, so I can answer only by analogy with writing non-fiction.  I've had extensive experience in writing about technical subjects for a mostly non-technical public policy organization.

The analogy might be: what do you start with: explaining the technology or explaining the public policy implications of the technology?  Explaining the technology is usually easier, but my audience wanted to get to the policy implications quickly.

I found that the way to start was to start writing in my head, then get words down on paper, then go where the words led me, and continually revise.  Other people constructed detailed outlines, and that worked for them, but it would have led to brain freeze for me.  In my method, I might have to tear up several pages -- they were no good -- or, more often, put them aside for a later section.   This sounds inefficient, and it would be for many people, but it worked for me.

Sorry, you just have grit your teeth and take the time to find out what works for you.