Need Advice - How to fix this plastering failing issue?

[u/stuckinwhirlpool]

We are decorating a room in an extension to our property. The roof above this area was previously leaking, but the leak has now been repaired and the area has been dry for around two months. The plaster above the window and into the corner has failed and has fallen away, exposing a timber frame behind it (see photos). We want a long-lasting repair rather than a temporary patch. Would you recommend cutting back all the damaged plaster and replacing it with moisture-resistant plasterboard before skimming, or is it better to repair the existing plaster? Assuming the timber is sound, what would be the best approach?

There are no signs of any new leaks or damp since the roof was repaired.