I just access my markdown files from Obsidian through nextcloud. When I'm on my phone I just use a simple markdown editor, when I'm on my PC I use Obsidian.
What I have gotten to work was to download a file from the sync app, open it in a markdown editor app and then save it to the cloud by sending it back to the sync app. It technically works but it was a bit too inconvenient to become a real habit (too many taps, need to rename the file on upload and set location each time,...).