With that in mind, this definitely feels like something I would do with a script. To make the script more compact and not have to hard-code all themes and their related keywords directly into the script itself, I recommend creating a [Themes] table. Each record would be a single theme, with the primary field containing the theme name. In a text field you could add all keywords for each theme as a simple comma-separated list. You would then link each incoming record in your main table to its appropriate theme record in the “Themes” table. The creation of that link could trigger a script that reads the related theme keywords from the linked record, parses the transcript, and updates a multiple-select field.
You might also consider adding another table called [Keywords], creating and maintaining your master keyword list there. Then in the [Themes] table, you could link to keywords instead of typing them, allowing you to connect keywords to multiple themes if that works for your use case. The multiple select field in the main table would then become a link field tying to this same [Keywords] table, and the script would update those links after parsing the transcript for a given record. You would then have the added benefit of being able to use count, lookup, and/or rollup fields in that table to track stats for each keyword if desired.
