The 20 gallon approach would work for you. I would not do the RO. Discus will spawn in tab water. If you would post photos of the fish. I can usually tell on sight a male from a female providing the fish have matured. Also, you can put 3 suspects into a 20 gallon tank and if one is of each sex then they will pair eventually. Chances in water will trigger spawning activity. Temp up or downn or a water change or a change in weather.will trigger if they are ready. There also may be a spawning season for these wild fish.