What's the best practice for non-90 degree corners on drywall / plasterboard?
Use scrim tape on both 90 degree and non-90 degree internal corners. I've never had a problem with it in internal corners, though I don't do much plastering.

You can use skim beads on non-90 degree external corners; it might help to squash them out to open further and better fit the profile of the corner.

Don't forget to scrim tape the straight board joints too in order to help prevent cracking in the future.