I decided to re-create my PDF Thumbnail Block using the free Genesis Custom Blocks plugin. Although there is a built-in template editor, it accepts only HTML plus the field placeholders, and this template requires PHP to retrieve the PDF thumbnail image.
Therefore the process required almost as many steps as using ACF blocks, though the plugin took care of actually registering the block.
Here’s what that block looks like.
Also, it appears the block accepts a class but not an anchor and there are no alignment controls. There may be ways to add those, but I don’t have time to look them up tonight.