There are quite some ways of achieving what you want to do.
Why not pass in font by reference?
Game(sf::Font & loadFont ,std::string title, int characterSize, sf::Vector2f position, sf::Color color)
You usually never want to load one font multyple times, and in here it seems that is the case.
Have sf::Font already loaded and checked for error, then whenever you need it you can use it.
sf::Font fontArial;
fontArial.loadFr...
//...
Game startTitle(Game(loadFont ,"Start", 24, sf::Vector2f(0,0), sf::Color::White));
Game startTitle(Game(loadFont ,"CrashAttempt", 32 sf::Vector2f(screen_size_x / 2, screen_size_y / 2), sf::Color(255,255,255,126));
But be aware, in order for your sf::Text to use sf::Font, you need to keep sf::Font instance alive.