Making onion soup made ahead of time

I'd make the soup base and refrigerate for up to 4 days. It will freeze fine (the base, with no cheese or other toppings) but you should check for salt when heating to serve. I'd probably splash some red wine vinegar in it as well, to brighten it up, as freezing can dull flavors. Whether refrigerating or freezing, I would wait until the day you serve the soup to put any croutons or cheese on top. ;o)

There's no definitive answer.

For sure the taste quality will decline before spoilage. Based on decades of experience storing cooked vegetable dishes, it is unlikely that I would attempt to keep this in my fridge more than five days.

I'd keep in the fridge if I planned on eating it in 2-3 days otherwise I'd freeze. Almost all soups freeze well if stored at the right moment (typically without any dairy for Western-style soups).

For sure if the soup calls for dairy to be added at the end, I would omit that whether I store in the fridge or freezer.

Best of luck.