What is the theme and a sub-theme for the short story "The Things They Carried"?
I think you are on the right track, but there are several interrelated themes of this story you might want to consider. For the complete information on themes, go to the links below and read all of what enotes has to say.
The things the soldiers carry in their backpacks reflect their individual lives back home. The heavy equipment they carry reflects the men as a group, depending on each other to try to survive. For example, Lt. Cross carries his stone as a connection to Martha and something outside of the war and Vietnam. When Lavender dies, Cross feels responsible and gets rid of those reminders that tie him to the world of home. The men must also share the burden of Lavender's death as a group and try to deal with it together.
Perhaps another theme you might consider is the one of illusion and reality. The things the men carry as individuals reflect their fears. The comfort those things give isn't real, as none of the things can really help. The equipment the men carry also give them an illusion of safety, but in the end, nothing could prevent Lavender from being shot. The things, both individually and collectively, give the men what they need to survive, emotionally and physically.
Hopefully, this helps. Good luck!