I'm not sure if this is an existing feature since I mostly use an app with Piefed. I like how some forums would alert you before posting if during the time it takes to write your post, some new comments were posted already.

A simple comment total check would do. And the ability to review the post in its current state before submission of you comment.

Not to demean your experience, but I get notifications as soon as someone replies to my posts, whether the page has reloaded or not. Has worked on both Voyager and browser. Are you asking for something like a pop-up over the "submit" button if there have been new replies?

Also there is definitely already a "preview" button

The feature would be for any post you comment on.

The preview option exists but not everyone uses it and sometimes one can forget to use it.