In Lightroom add an editable text box or two at bottom of Loupe view that can be Title or Caption. Could be part of Toolbar or not. The Metadata Panel is off to the side and somewhat busy to go through photos quickly adding captions, and nice to have panels off.
Yes, there needs to be some better way to caption photos. SmugMug used to have a view that you could just scroll down through photos and there were just title and caption textboxes that you could Tab through with a little thumbnail of each photo next to it. It made it so easy to add them. Also the ability to copy the Title to caption too.