Next step is to configure some plugins and global config;
this seems like it'd be a pretty common use case but although
some plugins have a separate config file, the majority of them
add stanzas to the main config.xml.
I can't see much to help me with managing 'fragments' of XML rather
than lineinfile (shudder). Has anyone had any success with this approach?
You can start with https://galaxy.ansible.com/list#/roles/2451 - this is a role showing entire deployment and config so you can get the idea, but there are other roles from the same user that deal with individual things (like jobs, plugins, etc.).