Following your advice, I've decided to use Excel, or rather a local variant (essentially just 10% gluteraldehyde solution). I hear that it is roughly 50% as effective as pressurised CO2, which is frankly more than enough growth rate for me.
Does this mean I could survive without raising the lights? The tank is two foot tall after all? Or maybe just remove one globe and leave only one running?
Plants can get 50% of their carbon from Excel. For every molecule of CO2 a plant uses, it can use one glutaraldehyde (Excel) molecule instead of another CO2 molecule. CO2 must be present for it to use the Excel.
In a low light tank with low CO2 demands, this can extend the small amount of CO2 absorbed from the air enough to make a nice difference.
But in a high light tank with high CO2 demands, the naturally available amount of CO2 is exhausted quickly, and any remaining Excel is then useless.
You're somewhere in the middle, and reducing your lighting to bring you closer to a low-light tank would be a good idea. If you can't physically raise the lights, then reduce photoperiod, or put some screening under your hood to block some light. Many two-bulb fixtures won't run with only one light installed, the only way to know for sure is to try.
Garden soil isn't really an option since I have to strip the tank and move house in a year. I'm putting fertilized laterite balls through the substrate where all the root feeders are, but that's it.
I see you were originally considering Flourite, which is just fracted clay with extra iron. There are other fracted clay products available, like Soilmaster Select and Turface; and even without the iron, fracted clay works well because it attracts nutrients from the water and holds them where the roots can use them. A 50lb. bag of Soilmaster Select cost me only $20 USD - considerably cheaper than Flourite. And because it's so light, 50lb. goes a long way. Though Flourite may not be readily available in Australia, I would be surprised if there wasn't some other fracted clay product that's available to you without overseas shipping.
Finally, following Tom Barr's suggestion for low-tech Excel tanks, I would need to change 50% water per week. That's a lot of water to be throwing out in a drought. Will my normal 20% suffice with slightly lower fertilisation levels?
Keep the fish load low so organics don't build up, and 20% should be adequate.