If you already earned the 1001% achievement on Steam but your in-game save file is blank, you don't need to download anything: Open the game and go to the screen from the main menu.
You spent 800 hours unlocking everything on your old PC. You bought a Steam Deck. You switched from PS4 to PC. You do not owe "the grind" to anyone. A save file restores your legacy. Binding Of Isaac Repentance 1001 Save File