Sloping ceilings 1980s ceiling

I’m dealing with a 1980s sloping ceiling constructed with 9 mm plasterboard, with about 2 inches of EPS insulation between the rafters behind it. I’m looking to improve the insulation performance of this ceiling.

At the moment, I’m considering three options:

1. Overlay the existing surface with insulated plasterboard (e.g. Warmboard).
2. Remove the plasterboard, seal the EPS sheets to timbers to eliminate the significant draft currently circulating in front of them (which is likely reducing their effectiveness through thermal looping), and then install Warmboard.
3. Remove the EPS entirely, replace it with PIR insulation, and then install Warmboard.

Option 3 seems like the best solution, as PIR has a lower thermal conductivity (i.e. better insulating performance) than EPS. However, the improvement may only be marginal relative to the effort and cost involved.

Has anyone faced a similar situation or made a decision between these approaches?