I'm only using my Nomad Checking Account for Exchange Remittances and to use my Digital Card on american websites and while I'm traveling. Do I have to declare the account?

Yes, you have to declare your Nomad Checking Account in your Income Tax Return of Brazil in the section “Bens e Direitos – Depósito bancário em conta corrente no exterior”. On your app, you will find a document that will help you with that on "Profile" - "Documents" - "Banking" - "Tax Reports".