Which badges are the reason why your users request to disable notifications? The preference only disables notifications for a few badges.
Maybe you could simply disable those badges. Then nobody will receive them and receive a notification. That’s almost like notifications disabled by default. I think it’s unlikely that new users check their preferences first to enable these notifications.
Do new users still see tips on your forum? I am asking because a Discourse update in August disabled the enable_user_tips setting.