Well, I think I've cracked it - after a few weeks of on-and-off work on making my webmention sending not spam everyone (see https://www.jvt.me/posts/2019/10/30/reader-mail-webmention-spam/ ) - it's now not re-sending them if they're successful. This is a good first step, but I'll be improving it to re-send if the post's data has changed since last time it tried (in the future). Note that this isn't quite done yet, expect it to be live tomorrow perhaps.
Something got me thinking about comments on my website here. Almost no one posts native replies on my posts. I’d have to think that 99.9999% of all the replies on my website are now via Webmention. Perhaps I should cut off native replies just to cut back on the amount of spam I get? Hmmm….