If someone buys this game on itchio, which I have, will the progress made on the itchio version be transferable to the steam version?
Yes, it should automatically use the same save files if you start the Steam version on the same computer that you've been playing the itch version on. If you really want to be safe you could back up all the files in %localappdata%/Circadian_Dice before starting the Steam version. Thanks for supporting the game!