The Places Details API only returns up to 5 reviews for a place. That limit is hard and there is no pagination for the reviews array. The next_page_token you are checking applies to paginated search results, not to reviews in a Place Details response. To fetch all reviews for your own verified business, you must use the Google Business Profile API’s accounts.locations.reviews.list, which supports pagination.

https://developers.google.com/maps/documentation/places/web-service/reference/rest/v1/places#Review:~:text=List%20of%20reviews%20about%20this%20place%2C%20sorted%20by%20relevance.%20A%20maximum%20of%205%20reviews%20can%20be%20returned.