I think this feature is already in qownnotes.

Surprised , nobody mentioned qownnotes

Edit: I was wondering how to enable this mode because it wasn't in my qownnotes ,Here's how I found it , go to the help section , click find action , and search preview and click on show note preview panel.

Now the caveat is that if you want to see it blitted , you have to save the file once to see it automatically show in the other side. Maybe this can also be definitely automated / I feel like there was some feature that did that for that as well or atleast its very non trivial.

Edit 2 : okay so I just realized that qownnotes also ships with autosave feature which saves and thus also shows what you type in reader mode in like a 0.5 second delay. And I think there is also a way to decrease / increase the autosave part as well

Dude , I didn't realize it , but qownnotes is so good!