For fermenting water with sugar, you need a complete yeast nutrient which basically gives the yeast everything it needs. Fermaid K and others, less komplex nutrients, are basically a mix of chemicals but they do not cover all the micro nutrients that are necessary for yeast growth. If you really need to add EVERYTHING except sugar, I would go with Fermaid O, which is, to my knowledge, the most complete yeast nutrient as it basically is dead yeast which has been grown to store nutrients. I am not sure if the Go-Ferm is really necessary for such lower ABV brews (Tosna was invented for wine and mead), but it certainly helps.
I googled your fermax, it seems to be diamonium phosphate and urea. So it is basically lacking everything else which the yeast needs. I would throw it away, tbh. as diamonium phosphate tends to leave a taste anyway. I would just buy myself Fermaid O and skip the go ferm.
I also would not use a champagne yeast, but a clean ale yeast like Nottingham or similar as it flocuulates much better than Wine yeasts.