Yes, if you get definite texture (rather than 'just a tiny bit of texture') in zone I, and you are interested in the ISO film speed, it could be up to a stop faster than your initial guess.(posted 9286 days ago)The zone IX is harder to guess. I can't estimate higher densities without a densitometer. The fact that it shows texture doesn't mean you are underdeveloping. Without a densitometer, the best test is: how do your negatives print? If you normally need a grade 2 or 3, all is well. If you normally need grades 4 or 5, then you should increase development.
If your air temperature is much below 20 deg C, then the tank will cool, which will reduce development, and hence contrast. Is summer in Australia that cold? You can easily test this before your next film. Just fill the tank with water at 20 deg C, and take the temperature after 10 minutes. The average of the two temperatures is (pretty much) the effective temperature of development.
Yes, a water tank will stabilise the temperature.