You essentially seem to want to be able to comment on a Post, which seems wasteful?
Also creating a layer of interactive comments below a Post level object might be a big development task imho (unless this already exists as a plugin?) and your budget is far too low imho. Consider all the additional administration you’d need of comments, both from user and moderator perspective - that’s a BIG can of worms you may not want to open.
As a workaround consider the following:
-
If you were to create a Category or Sub-category as “Topic”, then you could just permission the author to post Topics (equivalent to your “Post”). You may also be able to permission the author to create new categories (author is in his own group, you permission the group).
-
Then users could Reply to that Topic with Posts as “Comments”. Users would be in a group without permission to post Discourse Topics and would only be able to reply to existing DTs.
That delivers most of your functionality without any development work, just a bit of configuration.