I'm sure there are dosers that would work in a holding tank of RODI to remineralize it either with presoftener tap water or some more expensive chemical answer like Seachem Equilibrium or something. It can depend on how large a holding bin you'd need for your huge tank for a drip system or old style WC & where you'd put it...It sounds like you are a pretty handy DIYer...don't forget the need to heat the water at least in winter.
This makes me think of the discus people I knew in the East. I never really paid all that much attention to their water issues & how they solved them. I didn't have their problems or their willingness to fix almost any issue at any cost. Many used RODI but rather than drip systems many went for plumbed inline water change systems. Turn a stopcock to drain down to a drilled hole, then refill using a more sophistcated but similar idea to a toilet tank float. None had tanks as big as yours but many had more water volume in tank total amounts.
This makes me think of the discus people I knew in the East. I never really paid all that much attention to their water issues & how they solved them. I didn't have their problems or their willingness to fix almost any issue at any cost. Many used RODI but rather than drip systems many went for plumbed inline water change systems. Turn a stopcock to drain down to a drilled hole, then refill using a more sophistcated but similar idea to a toilet tank float. None had tanks as big as yours but many had more water volume in tank total amounts.