Quick take
For moderately hard city water, carbon plus hardness control may be enough. For very hard or inconsistent water, RO followed by controlled remineralization is often the most flexible setup.
Remove chlorine first
Chlorine can damage flavor, so carbon treatment is a useful baseline for municipal water.
Hardness controls scale
High calcium and magnesium create boiler scale. The right target is not necessarily zero hardness, but a controlled mineral profile.
When RO makes sense
RO is useful when source hardness or dissolved solids are too high, especially when remineralization can restore a suitable coffee profile.