If I’m reading your original post correctly, you are looking for a way for RSS to feed you only new topics, not every new comment. I note that TidBITS Talk has two RSS feeds:
https://talk.tidbits.com/latest.rsshttps://talk.tidbits.com/posts.rss
I suspect that the “posts” feed includes all topics and comments, and the “latest” feed only includes topics. If I’m reading your issue correctly, you want “latest” but have subscribed to “posts”. Hope this helps.