Hi davek,
In order to make a save file hard to mess with, you can encrypt the file.
Check out the open_encrypted() and open_encrypted_with_pass() functions at the FileAccess documentation.
(I can’t put any more links but I’d recommend Game Endeavor’s tutorial for savefiles: “Save And Load Data in Godot 3.2”)
Although the tutorial is old, the section that is about the encryption is still relevant as far as I’m aware (Starts at 9:30).
Also check out the feature to have savefiles as resources (.tres).