You mentioned that your water temps vary from 24˚C to 32˚C. So I'm wondering if the high water temps (that's nearly 90˚F) part of your problem, in addition to your hard water. Have you noticed that you lost a lot more plants during the summer when your water temps are that high?
It may be that the combination of your high temperatures & hard water will greatly limit your plant options. But there are plants that will do well for you in both hard & warm water. Many plant websites have lists of "discus plants" that do well in warm water, and some of the discus keepers on here may be able to provide some recommendations. My go-to plant website that lists hardness & temperature ranges is https://www.flowgrow.de/db/aquaticplants
One easy solution to help keep water temperatures lower is to direct a fan on your water surface to provide evaporative cooling. Some people mount small computer fans on the side of the tank, or you can just use a desk fan pointed at the surface.
Hope this helps.