I'd love to join!

From what I can tell, most of the time, the Sub-Conclusion is almost always going to be a Major Premise. This is because it is a premature conclusion that gives insight in order to offer support to a latter conclusion (the main conclusion).

Now, a Major Premise doesn't always have to be Sub-Conclusion. It can exist on it's own (just like we see in 2.1-2.10). You can tell a claim is just a Major Premise if it's sole purpose is to support the Main Conclusion and nothing more. You can usually tell if you can't identify it's own premises, unlike a Sub-Conclusion/Major Premise, which would give you a minor premise.

Let me know if that helps!