Maybe a janky way to add tags automatically for your database could be the Extract People and Other Metadata plugin (
https://www.mobileread.com/forums/sh....php?p=4287169)
I usually use my library for fanfiction, which comes with (messy) metadata tags created by the author to let readers know what to expect. If you could create tags for your library you could use tag:"=Theory" and figure out which files contained which tags by looking in the book details column.
This isn't really temporary, you'd be using this as a permanent replacement for the fulltext search. You would have to figure out how make different variations of the same topic into the same tag I suppose, but that would probably be easily done with some regex.