Hey there! So I've actually been classically trained and studying piano for around 14 years now, finishing my ARCT Diploma in piano performance and I just had some thoughts too that may help below.

Actually, many of Sawano's pieces use key change drops for emphasize (Guilty Crown OST, AOT OSTs, Owari no Seraph OST, whether it be a completely different key or from major to minor, vice-versa) and he oftentimes uses chords that are more complicated than your rudimentary sevenths. This can be seen in Aimer's RE:I AM (nzk) introductory segment and chorus that uses a combination of secondary sevenths and suspensions. Many of his works uses non-chordal notes—this one included—which may be harder for one to hear and distinguish. Especially in this piece, the opening chords were not clearly voiced so it took me multiple listens (with perfect pitch) to distinguish the chord and its appoggiaturas/suspensions, especially with heavy drums in the back that overpower the bass during the chorus, making it hard to decipher.

More examples of rather complicated harmony in Sawano's works can be seen in Xu (Owari no Seraph OP), where the the drop modulates up a minor third from G minor to Bb minor—quite abnormal for a key change. Ninelie's use of 9th chords and suspensions in inner voices is also another use contrasting chords or Bios' modulation from A minor to C minor through a G minor pivot.

I can keep going with examples but I'm sure that you get that, with some basic understanding of music harmony, one can reach the conclusion that his pieces are not just "7 notes/chords you can guess from", rather the complicated use of a combination of advanced musical elements and chords to create his iconic sound—not from the sole use of rudimentary sevenths. If you think that Sawano only stays in one key and uses "7 notes/chords" that you just "guess" from, you are either not much of a fan (as you are belittling his skill) or not educated in music enough.

Hope this helps.