Not wishing to discard them just yet, the next idea was to send chilled water in via an external cooling device. After an extensive search on Alibaba (for Ice Bath Machine), I settled on this compressor for liquids that has already been used once as an ice tub cooler in Australia.
This cost about USD 500 and additional for shipping, and together with some strong friends we lugged it to the balcony and figured out the basic plumbing to fit it up. Operation is as simple as switching it on and reducing the temperature setting to the preferred target temperature.
The compressor works by squeezing the refrigerant and with contact to the water pipes it will draw out heat to be extracted by the fan, just like an air conditioner's compressor. It does not come with a pump so I had to use a strong submersible pump to get water in the pipes and back out into the tub in a closed loop. Lots of visits to the local hardware shop to figure out the valves and fittings to make it work.
As you can see from the transparent box, this is a IP68 enclosure I got from RS Components that houses all the plugs that power the compressor, the water pump, the JED203 ozone generator (to kill bacteria), the Intex filter (to clean and suck gunk), as well as one remaining working chest freezer. All plugs are still wired to a WiFi smart plug so that I can control everything with my phone. Lots of fun wiring everything up, punching holes through the box using the cable glands to maintain the waterproofing capability, as well as extending some power cables via solder joints and shrink wraps (a heat gun was needed for this, more fun toys).
I punched a hole through the top of this chest freezer, removing the two aluminium layers and inner foam, then found the proper fittings for the hose to transit into a standard bathroom shower head hose. This will withstand the opening/closing of the hatch without folding the water hose. Folds become constrictions due to the heat on my balcony. This will be important to maintain flow rate when I decide to daisy-chain the tubs and prevent one tub overflowing due to random hose constrictions. Current temperature is around 11-13 Deg C, and I believe some cold is lost through the piping so I will be adding insulation between the compressor pipes and the tub. This compressor will be helpful for built-in pools and have the water quietly being chilled and cleaned a distance away, and the compressor and piping can be placed somewhere else. So far it's been an incredible project and I look forward to what the following years will bring in terms of experimentation and bringing more icy experiences to people in the tropics!