[stockkid]

Adding lines such as "topic:..." sounds like a very useful and low key solution. I might start trying that. Thanks.

You can use Dnote web app on the phone to do full text search and list notes by book. You can also install the website as a progressive web app for usability.

On the downside, using this web interface subjects users to a third party system (albeit open source) and forces them to give up the autonomy that they enjoy by simply using plaintext files.