I was on Onward three times last year and had difficulty spending remaining OBC.
Even the range of perfumes is very limited, but if your chosen fragrance is stocked you may be in luck. I eventually bought MoroccanOil products for my hair.
I think your 4 night cruise will be very popular, so you might consider upgrading your drinks package to the ultimate. This will allow you to experience the Atlas Bar and it should have a great atmosphere for that sailing.
Please let us know what you think of Onward once you return. For us, it has the same Azamara heart.