This is a great source with some good content written by scholars. If you find a topic that you want to learn more about, read the sources they cite. The form content is pretty good, but I would advise readers to go to the original sources (like Caplin’s Analyzing Classical Form: An Approach for the Classroom). The same could be said about many of the other topics. They provide good summaries and intros on open music theory.

You could use this part: https://www.teoria.com/en/exercises/ic.php. Teoria indeed always reverts around hearing what you do and reading it from formal notation. I think that's a reasonable design choice, but it of course goes beyond simply drilling intervals into you. To be honest, I used physical flash cards for the really basic stuff, like "What notes are in an E-major7 Chord?" I still have them and they served me very well. (:

Hookpad looks like a cool tool to register ideas. Or am I not getting it?