I believe if you set a modified speed (not preset) on the pump then disconnect power it will restart at that modified speed the next time you apply power. You can try it by setting the speed then flipping the breaker. When you flip it back on it should resume that speed. If that works then you can simply use your relay. As far as RS-485 comms with the pump Autelis makes a unit that will control it standalone.
1xVera3 1.7.619, 4xLinear WT00Z, 3xLinear WS15Z, 1xLeviton VRCZ4, 10xCooper RF9540, 1xLeviton VRFI10, 1xLeviton VP00R, 2xLinear GD00Z-4, 1xGE/Jasco 45612, 1xGE/Jasco 45610, 4xGE/Jasco 45605, 2xYale YRD220-ZW-619, 1xCaddx NX584, Autelis Intellitouch Pool Control, OpenSprinkler