Recently I updated my setup to use syncthing for syncing between my desktop, laptop, and my Android phone. On my phone I use Markor, an open source app that supports the Zim markup format (along with Markdown and some others). I've been pretty happy with this setup.
How happy are you with Markor on the phone? Is it good enough to edit your Zim files on the go or are there any bigger down sites?
I do have some issues making nested check box lists (maybe I need to review the Zim syntax) and it's not clear if you can add images. Mostly I review and update my todo items, read notes, and write small notes. For that I've been pretty happy with it.
And syncthing is great.