That’s indeed a workaround for new sites, and a possible template modification that could work for any theme. 
Old posts would need to be edited to replace the main image with a thumbnail in the post settings and a regular full size image in the post content, one by one, but at least with the added flexibility of possible image captions, without need to wait for custom fields.
However, maybe you need to fix some bug, 
since your site’s thumbnails are not being displayed at this moment.