At this stage I would just thank them for the well wishes and don’t mention the wedding at all. Since you don’t know any of the details yourself you don’t want to blurt out, “Thanks, but you’re not invited.”
Once you have firmed up the guest list and know who made the final cut and who didn’t you can tell people who aren’t on the list that you’ve decided on a small wedding. But don’t send out an email, only say something if someone asks you directly if they’re invited or not.
People are excited to hear about your engagement and are only asking about the wedding details to show their interest. It’s most likely not a way to “score” an invite.
When someone asks you about the wedding details, and you’re still figuring them out, be honest and tell them you’re just enjoying being engaged and haven’t started planning yet.