Sorry to hear one of your neons is sick and you lost a Cory! Great clear photos! Your fish has great colour!
I have had this with an odd neon maybe twice in five years, and confession time, I just euthenise the sick fish and hope it hasn't spread (and it didn't on both those occasions). I can't get out to do anything about it anyway, so unless it has spread I wouldn't spend a lot of time and money (vet/antibiotics usually, but I don't know what is available in Australia) on a £1 fish (sorry if that upsets anyone)? I have some neons (bought this last sunday) which are a replacement for the 6 (all) that died when we had a hot week in our summer and the water got to approx. 30 degrees on a few days and killed them all off - incidentally the Ember tetras survived in the same tank with out a problem. I think neons are pretty fragile/weak. If you can, I would try and keep the temperature at 24 but that may not be possible where you live?
I believe the bacteria that causes it is always present in tanks and only causes the lesions when a fish is stressed or weakened, usually poor water quality is the preferred culprit! Unlikely in your case though!
If your water is going from TDS 140 to TDS 200+ between maintenance/water change, do you know what is causing that? Do you get a lot of evaporation? I would think the weekly water change of 30% should be sufficient, though try and add the new water slowly.
The water parameters you have are obviously as a well researched BALANCE/Compromise for the variety of fish/shrimp you have and I wouldn't think they would be a problem! Neons prefer a lower PH and softer water in general I believe. How long have you had this setup running?
You may have already seen the below but I think it is a good overall article,
https://www.thesprucepets.com/columnaris-disease-in-aquarium-fish-1378480
Simon