I also have a Flysky I6X, which cable are you using? I think you need this cable to use the Flysky I6X: https://www.banggood.com/22-in-1-RC-Flight-Simulator-Cable-for-Realflight-G7-G6-G5-G4-p-950398.html?cur_warehouse=CN. Freerider works perfectly for me with this cable without any other software like SmartPropoPlus.
Another way would be using an Arduino board with a Flysky receiver and using vJoy to get the input, read this: https://steamcommunity.com/sharedfiles/filedetails/?id=1618655680. I'm doing this in Liftoff and works perfectly. but I'm not sure it works with Freerider too.