I agree^
I would experiment with cutting your tap water (pure source, not treated or softened) with RO. Ideally it will be a simple dilution.
While starting with just RO and reconstituting could theoretically be more consistent over the long run, using your tap will inevitably cut down costs since you wont need as much RO for volume or the chemicals to add constituents back. Assuming you don't see large swings in your source water it may be good enough.