I would get a smaller DWC reservoior. In your case; Let me give you a old time DWC tip.
As you know, you need to top off your DWC res’ with fresh PH’d 5.8 water every day or 2.
NOw let’s say that you use 20 gallons in your Res’. The rule on topping off says; " You can top off as much as you like between nutrient solution changes but, never more than the original amount of water), which means you can add up to 20 gallons of fresh PH’d water.
So; You could stretch out your schedule by seeing how long it takes for you to use the extra 20 gallons of fresh water. I would not recommend going longer than 2 weeks give or take a day, before adding totally new solution.