Set page template for a custom content type based on "Site Pages"

Hi all,

I am creating a custom content type that is based on Site Pages. It provides the selection of the page template:

However, using a custom page template here does not work. Depending on the permutation of the link, different things happen.

• /_layouts/15/SitePages/Templates/Wiki-Page.aspx -> Errors out
  
• /sites/team/SitePages/Templates/Wiki-Page.aspx -> Just opens the template as a page, not a new page based on the template.

This is very unsatisfactory. It would be a bit better if the default view of a template that is opened in the second option would at least have a button that says something like "Create page based on template". But it does not. From there, I can only edit the template. The only way to use the template is Home -> New -> Page. Very annoying.

How can I make this content type feature work so that the template is used correctly to create a new page?