Hello. I'm a newb here and I'm not sure if anybody has said this about this specific piece or any of your music, but if you want to focus on longer compositions, write for smaller ensembles or for piano solo or piano with a solo instrument or two. Then if you really think the musical material justifies it you can orchestrate it later for a larger ensemble.
I liked your symphony. It's wacky and unique. I'm not sure if in writing a symphony you intended to write something more epic and grand. For that you could write some well placed and orchestrated tutti sections. As it stands it's very linear and melodic but usually in a symphony there are also well placed accompanying harmonies to fill in the sound and make it sound more full. 1:41 in the woodwinds is a good example of that. I guess I just mean you could bring more meaning to your orchestration by having the melodic material require the orchestration to be more full sometimes and more sparse at other times. Same as you'd expect the melody to dictate when the instruments should play loud or soft I guess. I hope this post makes some sense...