I propose (and will discuss the details with the dev team) to run an in-game survey, so we get everyone's opinion.
From an end user perspective, the most effective way to survey all players would be at the login page, rather than in-game. Once the player logs in, they can be re-directed to a survey page, then given the option to either take the survey or skip it. If creating a survey page is too much work, you could use a third-party service like Survey Monkey, which is very widely used and might even be free.
