I've already built all the miners to mine an iron ore patch, but when trying to use the planner to place in the belts and electric poles, it decides that it wants to place it's miners offset from the ones I've already placed. (marking my miners for deconstruction and placing it's own miners) I haven't found a workaround for this issue so far.

Because that's that's not how the planner works?
It operates on the selected resources to design a layout start to finish, it doesn't take into account what's already built there.
I think you're missing the point of this planner if you're placing mining drills yourself and trying to use the planner to finish the job.