You started your ideas in a very solid and precise way, but then you became ambiguous.

If you affirm with so much conviction that publishing the same content in different spaces is "self-plagiarism" (which for me does not exist), then you should not argue that:

A simple blurb saying you are cross-posting is enough
This post has been previously posted on ...
(Please note that I have posted this idea, but this is completely fresh content)
Another way is to write an intro to your post, and then write:
You can see the full post here Making sure to link where the full post can be found.

Nor do I see how the chains are hurting. That explanation you gave about search engines is very weak.

Google and other engines don't know where the original content is, and who the copier is.

Friend, both posts are original. They are original creations of the author.

To finish, I simply must say that I do not agree with you.