Is there a way to have cards embedded into the post which will only be seen by members, or paid members? - for the rest will say “sign up or upgrade to unlock”.
No, it’s not possible to do this at the content block level because the post is rendered to static HTML on save. We have no way to know if the person who will eventually view the HTML is a member… etc.
Right now to solve this we’d have to store different versions of the post HTML for different membership levels, which will get infinitely more complex if we introduce more levels.
What you can do is custom logic inside of post templates.