I cannot sync steam cloud saves due to the save file structure in the Linux OS being slightly wrong. When you start the game, save files are created in .home/share/godot/app_userdata/Beat Banger, but also a save directory in .home/share/Godot/app_userdata/Beat Banger, much the same except with a capital G. This is because Linux folders and file names are case sensitive. The Steam Cloud files all end up in the one with a capital G, rather than the one with a lowercase G which seems to be the one the game uses as. I discovered that solution because moving the files from the capital G to the lowercase G makes the save work.