What should I add?
Looks great cactus! I would mainly tweak the lighting, maybe add an hdri for lighting as it’s a bit dark where the gun is and hard to see. A nozzle flare / fire from the tank would be pretty sweet so it looks like it’s shooting if you want more of an action shot but that’s just preference. You could also do a bit of compositing on it if you are familiar with that. Otherwise it looks great man! well done!