Can I also prepare the soup before serving (a few hours beforehand) and heat it up again?

I would absolutely agree with Gammy. It won’t take long to cook when you add the cream and salmon; most of the work will be done.

I would only prep and cook through step 2, where you add and cook the vegetables. When ready to serve, I would bring the soup so far to a simmer, then add the salmon and cream. You don't want to have the cream separate by cooking, cooling, then reheating; but most especially, you don't want to overcook the delicate salmon. Enjoy!