Can I make a game with one single game mode?
Is there a performance or functional purpose to having multiple game modes for the difference levels?
it’s completely up to you.
i only use one game mode for my single player game. There are a couple actual rulesets that my games “matches” can be evaluated by, but they are simple enough that defining it in a single game mode is perfectly fine.
be aware you can add actor components to the game mode as well, so it is easy to compartmentalize code if it was getting bloated without necessarily making another game mode.