Classical composers, symphonies

Some great discs with American symphonies:

https://www.amazon.com/American-Masters-Roy-Harris/dp/B000009CYH (Harris, Thompson and Diamond)

https://www.amazon.com/Ives-Places-England-Ruggles-Treader/dp/B000056TKF (Piston: Symphony No. 2)

https://www.amazon.com/Jack-Gallagher-Symphony-Ascendant-Reflections/dp/B00PO9AFSO (Jack Gallagher: Symphony No. 2)

I can agree with the OP's feelings toward Sibelius.
Besides the Violin concerto, and up to the 2nd symphony, I find him a bit dark- It must be those Finnish winters....
OP, if you like Schumann, then there are a number of Early Romantic composers.
Ferdinand Ries
Carl Maria Von Weber
Felix Mendelsohn
Ignaz Pleyel
Carl Czerney
Franz Schubert
Fredrick Chopin

Sorry if I have repeated some composers.
If you join Spotify, they have a nice catalog and you can search under a composer for a 'Related Artist'. I discovered many composers I never knew existed from that list.
Bob

As mentioned above, Shostakovich has a lot of big bold works.  His 5th Symphony is probably the most popular, and it's a doozy.  There's a great recording of the Royal Philharmonic Orchestra conducted by Charles Mackerras that I love, it's a great recording, huge dynamic range, and a phenomenal performance.  I believe this is the one, but I've long lost the original CD box so I can't remember what the cover art looks like:

https://www.amazon.com/Shostakovich-Symphony-No-Festive-Overture/dp/B000003YOH

Others to consider, especially if you're willing to wade into a bit of polytonality:

Charles Ives Symphony No. 2 (it may take a few listens for it to 'click')

Paul Hindemith's Symphonic Metamorphosis 

How about Schoenberg's Gurre Lieder! An impressive work - and sadly neglected! I remember having the Ozawa BSO version on 2 LP's! The same can be said for his Verklarte Night!

If we are including Hindemith, we should mention his most popular work: Mathis der Maler.

And since you like Mahler, you should definitely check out Shostakovich and Britten.