Is Sibelius using the current ASIO driver? If so, maybe we’re barking up the wrong tree…
Perhaps the drivers are good and something is up with Halion’s configuration?
Does Dorico come with the stand alone executable for Halion? ( I have a full version of Halion 6, so I’m not 100% sure how Dorico is packaged.)
If so, can you get Halion SE going outside Dorico (still looking for variables to eliminate…). If not, perhaps you can download the stand alone Halion 6 trial as an experiment. Sorry I don’t have any better or surer ideas, but this one has outmatched all my usual tricks…