Using a single-player 100% save is generally safe. However, using modified files to get impossible stats, or engaging in competitive online modes (like Ranked) with illegitimate progress, can trigger anti-cheat systems. Use at your own risk.
Before modifying anything, copy your existing save folder to a safe location on your desktop. If something goes wrong, you can restore your original progress. Method 1: Steam Version forza horizon 5 pc 100 save game upd