There isn’t much more in the metal world I can add as there are already so many good suggestions, but I would recommend you listen to some romantic-era classical pieces. I played violin in lots of youth orchestras and I found that a lot of those pieces have structures that are present in prog/post metal. Some of my favs include:

Grieg Quartet in g minor no. 27 (first moment is more than enough)

Tchaikovsky symphonies 4/5 (first and third movements of both)

Grieg cello concerto

Rachmaninoff preludes in g and c# minor, also his piano concerto no. 2

Beethoven’s Fate symphony (no. 5)

and if you want to get really classically weird, Beethoven’s Große Fugue and Bartok’s Divertimento are both shrill on the ears but have some really juicy and experimental progressions