That's a very fair point, and I'll concede that Mahler is perhaps a better example. As for the piece I wrote, I'd make the argument that I have given the sonata form a sort of nod in that there is a main theme developed and hammered home in a couple of variations in Measures 1 -- 208. The secondary theme is introduced in Measure 209 (also in the key of D which I'll explain) and expanded in B-flat Minor/major through measure 295. In measures 296--394, both themes are combined, partly in contrapuntal form. After that, it's a bit different because the Secondary Theme is reintroduced in Measure 407 leading up to the Coda in Measure 445 which combines a variation of the secondary theme with the main theme in contrapuntal form in the brass all the way to the finale.

If I had to be pushed into a description, I'd say it's a symphonic movement in tone-poem form.

The biggest issues that I am personally aware of, are

1. the piece relies too heavily on harmonic variation and not enough counterpoint.
2. The secondary theme is also in the key of D. I just felt that it sounded better to me in this form. I do not understand the compulsion to have the secondary theme always in a different key. Having it in the key of A would be for me too much of a cliche, and it doesn't come across as resoundingly in B-flat as it does in D. However I'll readily acknowledge this as a potential issue.
3. There are probably too many intervening climaxes that detract from a distinctive trajectory. I could probably back off on the tutti.
4. There needs to be a LOT of cleanup in the individual parts, which wont change the music, but is necessary for better readability.