You need to tell the user that, though, even if it's just a calming progress indicator or "Saved." label that appears a little after making changes, or a dummy

You need to tell the user that, though, even if it's just a calming progress indicator or "Saved." label that appears a little after making changes, or a dummy "Save" button they can click. People have spent years developing habits to defend themselves from crappy web forms. They're not going to let go of those habits without a long adjustment period.