I host a WordPress blog for my wife. She needs a simple editor app to keep a travel journal for that WordPress blog. Text and images. That’s all. Gutenberg is nice but way too many options and fussy interface. She wants a once-and-done solution for her iPhone.
Ulysses for both Mac and iOS offers an easy way to publish straight to a WordPress account, complete with tag, category, excerpt, and featured image support (if needed); you even have the option for Ulysses to post the article as HTML (the default) or as Markdown.
One thing you may want to look into is using Shortcuts for that—or possibly a text editor (even Notes) and then Shortcuts. The “post to Wordpress” action can be simple or complex, based on your needs, and can be set up to do exactly what you’re wanting. It’s not WYSIWYG, though, so you’d have to prep some HTML as a template first.
You may have looked into this already, but WordPress has an app for iPhone. Works on self-hosted blogs, and it’s got a pretty simple interface (at least it did last time I checked )
Byword is as simple as it gets. It has a clean interface that can be set to publish directly to the blog without stopping at the WordPress dashboard. Unlike Ulysses, it’s not on a subscription model.