I’m working on a web app now that supports Markdown and Org Mode with Dropbox:
https://ryanpcmcquen.org/DrWrite/
It’s completely client side code (running on GitHub pages). I just started on it, so no guarantees on robustness, but it does work reasonably well on mobile and desktop. It’s super minimal. I am planning to write a plugin for Obsidian that will add Org Mode support as well (using the same CodeMirror library that DrWrite is using):
Here’s the source code for that web app, BTW: