I'd like to see the source that says Dragon's WER in the 90s was 3-5%. I used Dragon in the 2000s and it just wasn't comparable to the current state of the art.
whisper.cpp is an opensource implementation, although I'm not certain exactly how open.
when you're providing context rather than instructions the tendency for a model to hallucinate or run off on a tangent is minimal, because the context you're providing has it's own cohesion.