Thank you @pamtbaau for addressing the question with Grav CMS
Right, I’m converting an existing site. I can’t use the same links due to bad practices, it’s something I need to correct. Anyway, I wasn’t just thinking about existing posts, but rather future posts. No problem.
I hadn’t thought about it until your reply refloated this thread. I didn’t know that different subfolders could be printed from the same parent folder. Grav surprised me again 
Having consulted the documentation, I think I have to use the following collection
content:
items:
'@page.descendants': '/blog'
Or with multiple collections for display only the posts, or using this other collection to display categories:
'@page.children': '/blog'
I look forward to testing. Thanks again!