What do you mean by android not supporting offline storage? Isn't it miles better than on apple devices? On IOS you can't access the file system, you can only send files from app to app, wheras on android you can access all your files through file managers and open them in whatever app you want.
I don't know a single writing app on android, where you can't save on the device and most allow you to browse all your files.
There are plenty off markdown writer apps on android. I don't use it, but for example
https://play.google.com/store/apps/d...erpad.x&hl=com should do what you want.