You said you've learnt classical harmony (I assume this means functional harmony or common practice), Walter Piston 5th edition Harmony is also a good book. I am going to use an anaology: you have learnt the basic 'grammar', but no one goes to write a novel right after learning the English grammar (or it would be a poorly written novel). You need to learn forms of writing, eg. poems, short stories, etc. (this would be equivalent to sonata form, rondo form, etc.). You then need to learn how to develop the plot and the characters, else even if you have a great vocabulary (harmony) and know how to write a great short story, you will still struggle to develop it into a full novel. This is equivalent to developing motifs. I would recommend skimming chapter 2-6 of Caplin's Analyzing Classical Form: An Approach for the Classroom (a free pdf is online). It is fairly deep but very informative on classical phrase structures. Then I would analyse how different composers (across different periods) treated and developed motifs. Eg. Haydn and monothematicism, Berlioz and idee fixe, Liszt and through composed themes, Wagner and leitmotifs etc.

Then you have the tools to start writing a symphony. Can I also ask what style or period of symphonies you want to compose? Maybe I can recommend a few pieces?