Control Panel for YouTube [1] has an option to add a new "Download" item to the menu in the video Transcript box (which I've just noticed is currently broken, as YouTube must have changed the implementation again recently)

Here's a fixed version [2] - run this when the transcript is open and loaded.

[1] https://soitis.dev/control-panel-for-youtube

[2] https://gist.github.com/insin/cb938324866c511066bcabe230b6a6...