I don't have anything to add to this particular suggestion. It's a good idea that clearly adds to SN in an organic way. I also really like that it emphasize substantive interactions, which is what makes SN so unique.

I did have a related idea for a nostr client feature that might be worth considering for SN. What if users could set these parameters for their content? If you really don't want to get spammed or see a bunch of lazy comments, you could set a higher price for commenting. If you want to leave a few comments on your own post, you can set that in advance. As you mention in "Risks and mitigations" there would have to be some baseline to protect the site, as well. Maybe there would have to be some extra cost of changing those settings when you originate the post.