Default Project Directory for the Launcher:
Edit the following file: %localappdata%/EpicGamesLauncher/Saved/Config/Windows/GameUserSettings.ini
Set: CreatedProjectPaths to path of your choice.
Default Project Directory for the Engine:
Edit the following file: %localappdata%/UnrealEngine/{UE_VERSION}/Saved/Config/Windows/EditorSettings.ini
Change {UE_VERSION} to match version you are using (4.23, 4.24, 4.25, etc.)
Set: CreatedProjectPaths to path of your choice.