Yes! Stretch ceiling will sit perfectly flat, and it never sags, bubbles, peels, flakes, moulds, or fades! It can be installed right over the sagging ceiling, or, if it is in danger of falling, you might choose to knock the existing ceiling out and then install stretch ceiling right over the empty space. You may also want to check to make sure that the floor above isn't sagging, in which case you may need to install a structural column or beam. We carry over 280 colours in high gloss, satin, matte, sandy matte, and canvas textured finishes, as well as translucent, iridescent sparkle, and we have the ability to digitally print any high resolution image on any of the above.

If the roof in the sunroom is also flat on the exterior part of the roof then perhaps put triangular windows in the top part of the wall that would look straight outside and then keep the sunroom ceiling flat. You wont notice its different as the window to the outside will be the feature of that wall.

Just a thought, have you checked your lovely flooring will persist under new cabinets or more importantly the gap created by the island bench. Possibly put a simple narrow breakfast bar (with power point of course) across a couple of the window sections, lovely to sit and look out of the window and preserve the look and the light. Then you would save some depth into the living space, as the current breakfast bar is quite large and deep.

For the best ceiling fan designs try Urban Lighting, they have a couple of shops in Sydney and some very unique designs in both traditional and contemporary. I have a deco house too and am looking at their lighting products for our new extension. The lines in your ceiling rose are sharp and square so I think a modern fan would look great, but luckily with deco you can go either way. The link is https://urbanlighting.com.au/collections/fans and below are some from their modern range which may compliment but not compete with the detail in your ceiling rose: