Can you send a campaign with a individualized greetings in the body of the sent
message. Like "Hello Francis" or "Hello Bob"?
Or is the only way to do that by sending 500
separate campaigns to 500 separate people?
Of course you can. Just put the personalization tag in your message and it gets replaced when you send it.
In your example, use "Hello [firstname,fallback=]" and put whatever you want the fallback value to be too.