Running both the new blog and WordPress on the same domain is not so much problematic but ... ugly. I can't set
blog.php (the new one) as the default
DirectoryIndex as it then breaks theme loading for the rest of the WordPress site that I actually want to use. I'll, therefore, just have to use PHP redirects from the Daily & Today pages. Not ideal but at least it works without impacting anything else.
When I move things over I will use the same filenames for the RSS feeds so that nothing changes from a reader's perspective.
This feels like such a big move. I have been using WordPress since 2008 (after I switched from my customised SharePoint solution, yes SharePoint if you didn't already know) so it's like a step into the unknown.