Finding the threads worth exporting
Start from engagement, not from a feed. Run the Reddit Search Results Scraper on your topic, sort the file by comment count, then export the comments of the top few threads. That is faster than reading a subreddit in date order. To take the comments of many threads in one go, pick Posts and Comments on the search or on a subreddit instead.
What one row looks like
One row per comment: who wrote it, when, the text, its score, the comment it answered and how deep it sits. The post's own title, text and score come with the run, so every file says which thread it came from. Is Submitter (OP) marks comments by the person who posted the thread, and Distinguished marks moderator and admin comments, so you can filter either in or out.
All Reddit scrapers are on the Reddit scraper hub. Comparing comment-export tools? See the best ExportComments alternatives.