Our game is a DOOM mod. It's not suited for minors but it's also not an adult only game. There is nothing inappropriate on the game page but DOOM has an ESRB rating of M (Mature 17+: blood and gore, violence). We provide an age check in our game for users that play online. We are considering changing the content warning settings and clarifying the age restriction and rating on the game page instead. Is there a better option? Is it ok to change the content warning settings after a game is published?

Yes, but it may cause the game to be taken out of search and placed back in the review queue.

I'm not sure, but for a game with gore and violence you should probably just add a content warning on the page instead of marking it as NSFW in the project metadata dashboard.