Insulation advice

Hello, i could really use some opinions on what to do or not to do. I have a classic 80s australian home, split level, upstairs is bedrooms with a normal ceiling cavity that is insulated. The lower split level is the classic vaulted exposed beam and rafter ceiling with no insulation. Hot in summer, cold in winter. A split system easily fixes this, but considering sometimes the temperature in the bedrooms is quite pleasant without aircon on, I wondered if I could achieve that downstairs as well and not use the aircon as much.

The downstairs is iron roof, on battens, on asbestos sheet, on rafters, on beams. Options I know of are create a 2nd ceiling between beams which maintains an air gap but loses the exposed rafters.

Or insulate between the rafters leaving some of the rafter exposed but then there's no airgap. Is condensation an issue in ipswich brisbane qld australia???

Or should I do nothing and just stick with the aircon and maintain the classic 80s look?