Principally I have been considering the Zinus memory foam, ZzzAtelier or Giselle mattresses. I have a $400 budget.
These are some examples of beds I've been looking at.
Zinus:
https://www.amazon.com.au/Zinus-Memory-Mattress-Pressure-Ant…
https://www.amazon.com.au/Zinus-Pocket-Mattress-Support-Cool…
https://www.amazon.com.au/Zinus-Euro-Pocket-Coil-Mattress/dp…
https://www.ebay.com.au/itm/Zinus-QUEEN-DOUBLE-KING-SINGLE-M…
https://www.amazon.com.au/dp/B07XG1CJ8X/ref=emc_b_5_t
ZzzAtelier:
https://www.ebay.com.au/itm/Zinus-QUEEN-DOUBLE-KING-SINGLE-M…
Giselle:
https://www.catch.com.au/product/giselle-queen-size-mattress…
I'm open to alternatives, however, they must be in a similar price range as I can not afford the Koala mattress, for example.
ZzzAtelier is better than Giselle. I got both 34cm thick models.