Have you thought about having placeholders in the middle of content representing a piece of encrypted text?

For example in viewing mode we see below:

The cash in bank is [encrypted] amount.

but when clicking on [encrypted], the app pops a password prompt, which receives a key as text. It uses that key to encrypt that piece of data and replace it with [encrypted]

When saving the raw content, and look at the raw content of the file we see

The cash in bank is [[piece_id=12345]].

by extracting all these encrypted pieces blobs to a simple file as encrypted_content_table.db which stores mapping of piece_id to encrypted_blog.

I'm not sure about how secure this is, but at least parts of the user content is not saved as raw text.