Thank you for this exercise. I will try it out.

Independently, before finding this post, I was playing with the TT1 block theme and the latest Gutenberg plugin. A Site Templates section appeared in the dashboard sidebar and I manually added some templates there. I also modified the header and footer templates via full site editing and the code for the modified templates appeared in that section, while the originals remained in the theme directory.

Will all those database stored templates go away if I change the theme to another block based theme? Or will they remain like custom field groups do when using ACF? It would be great if they could persist, since losing structure aka widgets is always an occasion for teeth gnashing and regret when changing themes.

Also, since the templates are not in code and can't be managed via git, will there be a way to export them so that they can be carted from a development site to a live site without a lot of copying and pasting?

I did like using the templates and the template idea of page building. It is very different from existing WordPress interactions however. It is as if WordPress is becoming a new product.