This is the correct thing to do. Using the since and before parameters, you can continue to page through results to get everything. There is a brief mention of paging here: https://developers.trello.com/docs/api-introduction#section-paging
It is also worth mentioning that if you’re trying to do a sync (that is, grab the current state of a Trello board and then only get stuff that has changed) the cards_modifiedSince on the cards nested resource may be useful to you. It accepts a timestamp or an ID.
Not yet!