Sunday Closures

You have posted this in the forum about the Netherlands, but in your question you mention both the Netherlands and Belgium. Which country does your question relate to? Or will you spend a Sunday in both the Netherlands and Belgium?

I found many restaurants closed on Sunday in Ghent Belgium. I cannot speak to other places in Belgium.
I arrived in Amsterdam on a Sunday and did not notice stores being closed.

In both countries in general hardware stores and fashion shops are closed on Sundays. But for the latter in larger and touristic places not so much. Larger supermarkets like Albert Heijn and Jumbo in the Netherlands are open everyday, with the exception of the Dutch Bible Belt where most if not all shops are closed. But that’s outside Amsterdam in area’s lesser visited by RS travellers.

In Belgium the larger supermarkets outside the historical centers like Carrefour and Delhaize are closed on Sundays, but their smaller branch stores like Carrefour Express and Delhaize Proxy in the historical centers can be open until noon, some open the whole day. The same for bakeries. Typical tourist shops in Bruges are open every Sunday.

Wouldn't worry too much about restaurants.