Bienvenu, mon ami. Je pense que je continue en anglais... My now mostly-forgotten high school French will not hold up in scientific discussions.
RO units are not inexpensive, but long-term they will do the job with reliability. If there is no major industry where you live, rainwater is another option. But of course you need a lot to do weekly water changes regularly. Too bad we can't get a pipeline from Vancouver to Quebec for you
. Except for the low temperature, you could put discus in Vancouver's water reservoirs and they would thrive; zero hardness and pH in the low 5's.
Water quality is equally important to actual volume. Jack Whattley has frequently written of this, how it is possible to raise discus in smaller tanks with daily 100% water changes. That was an experiment of course, not intended as standard aquarium husbandry.
Swords actually take a lot of nutrients out of the water and are good filters. Pennywort would be a good stem plant; my luck with hornwort was near zero years ago. And floating plants are essential to dim the light and calm the fish, plus being floating plants they are very rapid growing.
You should talk with Kym (Aunt kymmie), she has discus in her 90g, and could tell you how she manages. Her water is not soft if memory serves me, but as I said before discus can be maintained in basic water, it is the rams and cardinals that are less tolerant, and by that I mean they last for a time but no where near their life expectancy of more than 10 years. The minerals in the water cause calcium blockages in the kidney tubes and suchlike.
In their native waters, cardinals are annual fish, only living one season; there is insufficient food for them to last after they spawn. A few years ago there were exports of more than 16 million cardinals every year from Manaus, Brazil, and still the wild population every year was as plentiful as the year before. The aquarium trade actually did good by removing the fish that would otherwise starve to death. Unfortunately that is not the case with many other species, some are now near-extinct.