Please switch to Desktop mode to transfer your save data from demo on the SteamDeck.
1?Access the following directory in Desktop mode and copy the files in the "_data" folder:
\home\deck\.local\share\Steam\steamapps\compatdata\3347730\pfx\drive_c\users\steamuser\Documents\MyGames\FANTASIAN Neo Dimension DEMO\Steam\?SteamID?
2?Paste the copied files into the following directory (if any data for the full version exists already, delete those first):
\home\deck\.local\share\Steam\steamapps\compatdata\2844850\pfx\drive_c\users\steamuser\Documents\MyGames\FANTASIAN Neo Dimension\Steam\?SteamID?