RDWC without a dosier could be tough and water temperature changes can mess with the PH too. I have a reservoir for my coco auto feed and another for my Ebb & Flow that dont have dosiers but once I dial it in the PH usually holds, if anything the PH goes up. The RDWC has the doiser and the PH goes down down so much I will use a gallon of  of PH up in a grow.

If your having trouble with RDWC then I dont thing DWC will be any easier. In the long run I think coco coir and an auto feed system is the way to go.