The answer is that this is not a concern. My trim pieces will lay flat against the sheathing. I don't need them to sit flush against or even touch the jamb.
I'll just have to use a lot of caulk to fill that gap. I am also going to look into those quarter round, thin moulding strips. The header trim piece is also not a problem, because it has the flat surfaces of the side pieces to sit on.
Failure of imagination on my part. I'm tempted to delete this question, but maybe this will help some other fool out there like me. Thanks for the feedback, Jack!
Edit: I did get some thin molding strips to go into the gap.
