Markor is an app for "notes", that just happens to be fully capable of using markdown, and can handle mid length writing (up to a couple of chapters).

While it doesn't sync with its own service, you can use any syncing apps to handle that. I use foldersync on my android devices, it syncs both ways from gdrive, my nas, and the sd card on each device it can be scheduled any way you want.

But, when you know you're going to get wordy, you open markor, type away, and do it at leisure since unless the device dies totally, it'll just save where you are if you close the app. When you're done, copy/paste back into your app/site and Bob's your uncle.

I use markor because it's well coded with no bugs I can find, it's open source, and it bloody well works. Standard markdown formatting, easy exports, easy to use. It's pretty much the most reliable app I've ever used, and by far the most reliable note/writing app