Hi @nugley,
Ceiling plasterboard often cracks at corners and direction changes because these areas carry the most stress as the building moves with temperature, humidity and settlement. Any small movement in the framing transfers force to the plasterboard, and joints at corners are the weakest points, which makes them a likely point for cracking.
For this reason, installing a Plaster Expansion Joint across that middle crack is certainly going to be worthwhile and will have the most effect in minimising future cracking. It will allow the plasterboard at the corner to flex with the different movements of the framing.
The top and bottom cracks can be managed by v-grooving them with a utility knife to open them slightly, ensuring you wear gloves while doing so, then applying mesh tape over the groove and coating with Multi-Purpose Joint Compound before sanding and painting. These cracks are less likely to recur with the installation of the expansion joint, and the mesh tape will help to strengthen them to prevent further cracking.
Alongside the expansion joint and crack repairs, your efforts in screwing and gluing the plasterboard back to the joists will also help to reduce unexpected movement, which is a cause of cracking. This, combined with the expansion joint and mesh tape repair, is about the best you can do in this situation.
Let me know what you think and if you have any further questions, please let me know.
Jacob
