Quick take
For many homes, carbon filtration plus moderate hardness is better than zero-mineral water. Very hard water may justify RO followed by controlled remineralization.
Remove chlorine first
Chlorine can create obvious off-flavors in brewed coffee. A good carbon filter is often the simplest upgrade when the source water already has reasonable mineral content.
Hardness is both useful and risky
Calcium and magnesium contribute to extraction, but excessive hardness creates scale in kettles and espresso boilers.
When RO makes sense
RO is useful when source water is very hard or has other dissolved contaminants. For coffee, the resulting water often benefits from remineralization rather than being used as ultra-low-mineral permeate.