I wouldn’t add a button there. The canned-replies plugin was like that. IMO, the interface gets a little bit cluttered for something you woudn’t use that often.
Maybe it would be better to create a route similar to /top or /latest that would list all available templates in on place. We would have maybe /templates and there you would just have to click on the topic to open it.
I thought about this when developing the plugin.
I know it could be convenient, but in my opinion it causes more problems than it would solve.
Topics tagged that way can’t leverage the category permission system to define if the user can use templates or not and at least for me templates should be in a place (category or tag in the case of the proposed personal templates) where anyone who deals with them knows that it is dealing with a template.
Imagine you tag a post from a third party as template and it gets edited to something you don’t expect. You will be unaware the template was changed and the person who edited it will be unaware it edited a template.
I don’t know, in this case I prefer a well defined flow.