Proof of Concept User Interface for WordPress Block Templates.
To create a page template just layout your blocks the same as you would as normal. Use the meta box to indicate where the template should be used, and then when you create a post the template will be applied.
Note how blocks can be locked. A PoC plugin exists to restrict this further so a block cannot be changed at all based on user role.
To update a page template, simply amend the template. The next time you come to update a post that uses it you will be advised of updates, and WordPress will apply the changes without impacting any changes you have made.
By default the templates are locked, meaning you cannot change the blocks on the post that uses them. This should be an option when creating the blocks.