I have second thoughts about this and think it is too complex for the benefits it brings. I still think that we should move from the regex site setting, but we can just introduce a new watched word “Silence” with a description like “First posts of users containing these words will require approval by staff before they can be seen and the user will be automatically silenced”.
Even the terminology of “first post only” is a bit confusing because it may suggest that those with “first post only = false” will apply only to second, third, etc. posts, which is not true. They still apply to first post and this is needed to replicate the existing functionality (first_post_only = true → only the first post, first_post_only = false → all posts).