I have a 425 gallon system, with two 100 gallon storage tanks, one for salt water and one for RODI water. My RODI system keeps the RODI tank full. There's a single pump with valves in the plumbing to control the source and destination of where the pump pumps. So I can pump RODI into the mixing tank, the mixing tank into my system, and either RODI or mixing tank water into a spigot located next to the mixing station.
There are two large rectangular fiberglass (I think?) tanks for the water - the mixing tank sits on top of the RODI tank. So you can't get into the RODI tank, but there is an opening on top of the mixing tank for storing salt.
I have a GHL Maxi doser I use for daily auto water changes. One pump head pumps water from the mixing tank into my sump, and the other pumps out of the sump into a drain. As I said, I can also pump at a higher rate directly from the mixing tank into my sump, but I only do this if I'm doing a very large water change, which I rarely do.
This setup works well for me because I have a basement sump. It sounds like you want your mixing station in the basement and your display tank and sump are upstairs. With the right dosing pump, you might be able to do auto water changes similar to how I do it. If you want to use a larger pump in the basement to pump upstairs, you could probably remotely control the power to that pump by plugging it into a wifi controllable outlet, and also have a valve upstairs to turn the flow on and off.