Update
My first piece is ready now so I wanted to know if the second part is as good as the first part and what i could do to make it more interesting for the musicians and the audience. Maybe I need something for the cellos and the second violins because they only have long notes, but too many notes will make the peace sound not calm
Comments
Your conclusion was perfect. You've asked what can be done to make it "more interesting" and about "orchestration." Answers would be subjective to a person's taste; there is no right and wrong.
Too many notes? Observe all the notes in Debussy's Afternoon of a Faun. The tranquillity is not disturbed. Your "calm" atmosphere is due to the harmonies you have wisely chosen, as Debussy did with his new language. Another choice of harmonies, even at pianissimo, would introduce the opposite effect.
The 'cello line was good. I especially like the 2nd violin line at bar 56. This is where subjectivity enters. There is where I would prefer a viola. However, your second violin is okay, but its line is emerging out of the space where the 1st violin accompaniment speaks which weakens the counterpoint a bit. A primary difference between instrumentation and orchestration is the exploitation of register. A subtle register change in either the melodic line there, or the accompaniment, will (I believe) give you a more convincing result.
If I may, your opus 1 gives you a great start. It is a complete musical thought, well enunciated. I'm not one who likes to give unsolicited advice, but since you've asked...
I'll thank you for putting up with me.
(Oh...small point: don't forget the second "t" in allegro affettuoso. The Italians do pronounce both of them.)