Judging by the plan, the photos and your comments it appears that too much heat is coming in from high light glazing (clerestory windows) into the lounge and dining areas. Both rooms will be cold in winter because the direct heat from these windows is not enough and not absorbed in an thermal mass and they do not have ground level windows or glazed doors facing north. The elevated floor means that it is cold under the house so heating in winter goes down as well as up. The roof would lack adequate insulation due to the period in which the house was built. The roof tiles probably have another 15 years maybe a bit more so it is costly to insulate from the top (i.e. taking roof tiles off adding insulation then putting roof tiles back in again. The colour of them is contributing a lot to the heat. So what to do. - add external shading device to the highlight glazing, maybe timber battens 40 x 40 at 20mm spacings. This will block direct sunlight within the shading season. That helps to reduce heat intact and see how you go. If that does not work then - add a rigid foam core board to the underside of the timber ceiling, e.g. Foilboard which is 20mm thick r 2, then install a 12mm plywood ceiling lining under this. That way you retain the timber rafter ceilings but also one that has a light coloured (hoop pine ply) timber in between. (Note this is costly) - big gains in heat loss is achieved with underfloor insulation. Try r3 bulk squeezed tightly between the floor joists. Then you can plan for the future with the following - change glass in windows and glazed doors to a low e glass - install insulation into the cavity of the external walls using a company called envirowall. - replace roof tiles with a light coloured metal roof, replace the existing roof battens with a 100 x 40 batten so that you can install a 100mm thick anticon insulation. All of the above costs money and if you don't do it then it costs money to burn energy to keep the living rooms between 21 to 27 degrees year round (what we consider comfortable) so if it tough to decide what to do or not. As an alternative For about 40k you could spend 30 of it on solar cells, and an awesome battery (Tesla powerwall 2) and buy some wall mounted ac units. But that would be kicking the can down the road. But sometimes you have no choice. Good luck

I was in a similar spot with my renovation a few years ago and ended up going with engineered wood floors in a dark oak finish for the main living and kitchen area. What really sold me was the maintenance, so much easier to clean after kitchen messes, and the boards still looked good even after a few scratches from the kids. I found the better-made engineered boards with a thick top layer can be refinished at least once, and they feel pretty solid underfoot. In my hallway, I went with solid timber as a bit of a test; honestly, both have their strengths, but the engineered option gave me more of the color and look I wanted with less stress about upkeep. If you’re leaning towards a dark timber look, I found a lot of inspiration just browsing different styles at places that specialize specifically in natural wood floor, places that let you see the real thing laid out. It definitely helped me make the final call.

start with checking all your ceiling bats and adding double glazed windows in the main living areas and add insulated curtains (double layered not rubber backed) to all windows and zone inside heating areas with solid doors (that are kept closed) across hallways so that you only heat the rooms that you're using....we replaced a 60 year old damaged tile roof with steel as it was impossible to match old tiles for an extension and steel was cheaper than new tiles and painting and repairing the tiles was not much more cost effective but, unless you have leaks or broken tiles, there should be no need to swap tiles for steel and adding best rated bats over the ceiling should make a huge difference and is a simple job for an insulation company (or a tedious job for a homeowner) but i would first have all ceiling wiring checked and any rubbish cleaned out of the ceiling cavity and as already suggested check yourhome.gov.au for advice for retrofitting insulation and other ideas to improve your existing home with passive solar design

Thank for the comments. I appreciate it. Yes, I'll be pouring concrete onto existing concrete. I'll call my local council for advice and see whom else I need to speak to. It is a habitable space, but raising the floor will allow minimum ceiling height requirements to be maintained. I'm currently getting the original building plans from council and will be showing them to a structural engineer for advice on the house's slab and whether the additional load is ok.