coffeealex “Mineral content” (TDS or dry residue) and “probability of scaling” are not the same thing. You could add a lot of sugar to water (more than the weight of water!), and it wouldn’t scale at 130 °C. Heck, you could add quite a lot of copper sulphate, and it wouldn’t scale.
For scale to form you need Ca and/or Mg and (bi)carbonate (or sulphate, but far less likely) ions in solution. If you only add bicarbonate of sodium or potassium, there is no Ca or Mg, so scale will not form.
Flushing is still required as at some point, as bicarbonate salts convert to carbonate with heating, and carbonate solutions can be corrosive. Do you need to flush frequently? No, probably once every few months - and that’s largely out of caution.