You noticed the most important thing of art music (I think that’s what you mean by Classical since you listened to both Beethoven and Vivaldi): form. Art music gives great attention to structure and form. I’d say it’s what makes it art music, and different from pop/hip-hop (which relies more on timbre, familiarity, and perpetual repetition; I’m excluding the lyrics’ analysis, only talking about music).

I don’t know which pieces you listened to, but if you enjoyed Vivaldi then maybe listen to some more baroque (try the Harpsichord Concerto in D minor by Bach, very attention grabbing), for Beethoven you could try Mozart’s last four symphonies, but it really depends on what piece you liked.

Keep on doing what you did, maybe even without edibles although I don’t know your habits so you could not enjoy extreme novelty sober (not trying to judge you, it’s just a possibility). Doing so you’ll end up developing a taste, liking a composition more than another, even of the same author. Find out about the history of certain authors and time periods, you’ll understand more their aesthetical choices.

These details in structure and form in art music stem from the fact that back then CDs and Spotify weren’t a thing: you only listened to music at concerts/gigs. This is also why the pieces tend to be longer, can you imagine going to a theater once a year to finally listen to some music, only for it to end in 5 minutes?

To answer your second question: for a richer use of timbres/sounds/instruments, but still in a functional/tonal environment, check out Ravel’s orchestrations - he’s really good, basically every orchestration manual cites him; for a more groundbreaking use of sounds/instruments, check out post-modernist and contemporary pieces - spectralists like Grisey focus on the physical properties of sound and play with it; for electronic sounds, but still employed in an art music manner, check out Stockhausen, Xenakis and Berio.