From reading this article I decided to create a post-show.php file and point to that in my get_custom_template function. Then, I took the meat of the post-show template – essentially everything but the header and footer – that I had created in the full site editor and created a template part called “show” out of it. Then, in my post-show.php file, I added all the other parts of the page, called the header and footer using block_header_area() and block_footer_area() and finally added the “show” template part using block_template_part().
It all works. But this largely defeats the purpose of full site editing in that I will have to go in and change this php template file should I want to add other parts to my posts outside the “show” template part (and it would be even worse if I was allowing other users to edit template files in the full site editor). What I think that we would need is some sort of block_template_part function for entire templates. And I am still hoping there is some other solution that I am not yet privy to.