Since you have a GT subscription you can watch my tutorial that is an overview of music theory topics.

https://www.guitartricks.com/tutorial/495/

Or this entire collection of tutorials.  This starts with the overview tutorial and goes into more topics.

https://www.guitartricks.com/collection/music-theory

I don't have any experience with the Beato book, but it's probably worth a look.

The gold standard for academic music theory (non-instrument specific) is Tonal Harmony by Stefan Kostka.  You can find inexpensive, used copies online.

https://www.abebooks.com/servlet/BookDetailsPL?bi=32362836293

I've also seen them in used bookstores in bigger cities (Memphis, Nashville, etc.).  I've worked through it a few times and it is outstanding as well as comprehensive.  But it is heavyweight material.

Alfred's Essentials is a little less heavy.  But also a good primer.

https://www.amazon.com/dp/0882849514/

Finally, I get that you want something musically productive to do away from the guitar.  And I love studying & discussing theory!  So I definitely encourage students to take an interest in it.  But I also always encourage students to be aware that it is absolutely essential to directly apply their music theory learning in order to make it useful.  Your understanding of theory and practical playing skill should grow roughly together, at the same pace, so they are mutually reinforcing.

Hope that helps!