This would be consistent with standard wikipage editing, wikitext and VE, which both use Save page.

VE is not a valid comparison, since it's not designed to work for talk pages.

For wikitext, it doesn't have access to the same information. The 'new section' form (https://en.wikipedia.org/w/index.php?title=Talk:Earth&action=edit&section=new) does know you're adding a section, so it could say 'Add section', but that doesn't mean we should necessary do it the same way.

However, wikitext never knows whether you're replying, whereas we do (and 'new section' is similar but not identical to 'add topic').

We may want to discuss this more, but this seems like a benefit of Flow, rather than a place we want parity for parity's sake.

The preview bug is valid, but we are already tracking that elsewhere.

I agree it's good to include "topic" / "post" in the button label, since Flow knows which one it is. But I still think "Add" is ambiguous. After opening the edit box and typing some text, the user has already added something to the board as they currently see it, and they might interpret an "add topic" button as "open another edit box for adding one more topic to what I already have".
The button's action is to send the addition the user created to the server, and the user shouldn't need to know that this is technically adding a new entry to the database rather than updating the stored version of the board.
One could also interpret "Add" as the whole sequence of opening the edit box, filling in text, and submitting it (this is how phabricator does it).

To be fair, "save" is slightly ambiguous as well: in Blackboard Learn's discussion system (which many students are used to), you have a "save" button that saves the post privately for later editing, and a "submit" button that actually posts it to the board.

I would think "submit" is really unambiguous, but here the question is if that's maybe too technical.

So my preferred version is "Submit topic" / "Submit post", followed by "Save topic" / "Save post".