I’m leaving my idea here to see if it helps. You don’t necessarily need to convert this to Google Sheets.
Your comments are writing to the App: Comments sheet. To make it visible to Glide, use an Arrayformula like.
This will copy the data to another sheet and make it a table in Glide, of course this comes at the expense of a lot more rows.
Next, assuming your comment has a relation to the post by the post’s row ID, create a multiple relation from the Post sheet to the Comments sheet, return the latest timestamp of comments.
In the inline list of your post, add an action that sets the “current timestamp” to a user-specific column whenever your users views a post.
If the “last visit” is before the latest comment timestamp then that post can be marked as “with unread comments” or something along those lines.