I know CBP allows you to amend your declaration until you're caught. Does CBSA do the same? Currently on mobile so I can't check the Customs Act. According to the website though:
If you are unsure whether to declare an item, or whether an item may be inadmissible, always declare it to the border services officer. Officers will assist you in making your declaration and will work out your personal exemption and any duty and taxes you owe in the way that benefits you most.
https://www.cbsa-asfc.gc.ca/travel-voyage/declare-eng.html#_s14
Having said that, as a Nexus card holder, you're supposed to know the rules. When you have to fill out a declaration card, you need to fill it out completely -- which it doesn't sound like you did.