Yes, that would be nice, although I'm not sure how implementation would work. You need something to anchor the toots to the post. The logical choice is the URL of the post. However, this can be accomplished already with webmention and doesn't need ActivityPub at all. You have to use something like Bridgy to monitor your account for your domain and then pass them to webmention. Unfortunately, Bridgy can be a little finicky.

I used to use a WordPress plugin that allowed people to post comments using their social media accounts, but that was just for verification and it wasn't an integration with social media.

My understanding is that followers will only see content from the time they subscribe/follow forward. As Zak mentioned in another comment it's not that they can't, but the platforms choose not to. That said, I don't use the website as my daily fediverse account for a number of reasons. The integrations I use allow for comments, likes, and boosts to be captured, even if they're not replying to a post from my blog account.

Also, if someone looks up my blog on Mastodon they get a message saying previous posts can be found on the original site with a link to my author page.

I agree, that the implementation has its issues, but it's a start. Version 2.0 is supposed to drop in a week or two, which brings some needed enhancements. I didn't realize that the lack of post history was Mastodon-specific. However, when I try to view my blog through different sources I have the same issue, so maybe that's a standard practice?

For some reason it posted the image and not the link. I've updated the post, but here's the link as well. https://jseggers.com/technology/how-to-set-up-activitypub-for-self-hosted-wordpress-sites/