Pursuing Theoretical Computer Science after CS major
In order to get some feeling of what TCS might look like, I recommend watching the lectures given by Prof. Ryan O'Donnell:
https://www.youtube.com/c/RyanODonnellTeaching/playlists
And lectures given by Prof. Tim Roughgarden:
https://www.youtube.com/channel/UCcH4Ga14Y4ELFKrEYM1vXCg/playlists
And by Prof. Jeff Erickson:
https://www.youtube.com/channel/UClzjdrM8L81bVX_a6u6Hobg/playlists
You may also want to browse some of the podcasts by Simons Institute:
https://www.youtube.com/c/SimonsInstitute/playlists
Good Luck!