I just came by Obsidian app which is a note taking application available on all platforms. I found that Obsidian is really good for writing notes. Notes are saved as markdown (.md) files in the local file system. These appear when we connect iPhone to the mac and we can easily backup by dragging and dropping the folder. I read that there are plugins for syncing.

Update on 15 June 2023
I have migrated all my notes to Obsidian. Now the app is taking more than 20 seconds to open an offline vault containing around 300 plain markdown files. This is unusable for me. Back to DEVONthink To Go.