Piano Forum

It would be a huge help if you could start with piano reduction if it exist IMO. Simplifies a lot. I cannot imagine how to analyze a line if you cannot hear it in your head, and that means you need to know well how an instrument sounds.
Each symphony is a form of drama. One suggestion would be to identify themes (usually in different keys) and melodic repetitive motives. Repetition of these motives is frequently if not always one of anchors that keep things together.
Think of the themes as songs - say, two main characters sing and speak through the piece, with more characters possibly added later.
When the music progresses, the themes progress as well, new theme(s), i.e. characters, may get introduced, existing themes usually change its key few times to change the narrative (from main charaters falling in love to excitement to tragedy, etc.). And in the end themes may go back to an initial key.
There would also be intermediary movements - dances, etc.
Reading about the specific symphony creation history would help in understanding its dramatic narrative.
There are theories and books on orchestration and analysis.
Id say that classic opera, ballet, symphony are all built similarly, but voice and dance are easier to interpret when you listen.
But all I wrote is not about deep analysis, it is more about general composition. If you want to see what deep analysis is, start by watching Youtube series by Jacob Gran on Schenkerian Analysis - one of analytical approaches.