@pfefferle See my other reply for details and an example.
I can get around this by reposting the URL from this account (not associated with the blog) and adding the hashtags in my content. But that's tedious.
Or I can wait for the long-promised #quotetoot feature (or whatever it will be called) to be added to Mastodon's core code, but even then I would still have to reshare everything and manually add the hashtags.