Solution available and widely verified by the community. Risk Level: Low (if using raw save files).
There is no official "all cards unlocker" from Konami. The game originally required you to defeat in-game opponents (like Joey, Yugi, Kaiba, Mai, etc.) multiple times to earn DP (Duelist Points), then buy random card packs in the shop. Some cards were extremely rare or only obtainable after specific story progression.
Ensure you have opened the game at least once so the registry entries are created. Apply the Unlocker: