@MiaSkywalker It turns out the Launcher/Studio page just didn't like my custom thumbnails I applied using the Sims 3 UIC tool. It has seven thumbnails, so replacing them all may have made the file too large for the process to handle. Although, I don't get how it can have limits, since content itself can enlarge a file indefinitely. Regardless, it has been uploaded to My Studio now. CC free and ready to go.
Thanks for all the pointers and tips, even if all they ended up doing was making me look where I should have looked first. It was my first time using UIC, so that should have been my first suspect.