Atlantis is pretty active in putting out various discounts, but peak weeks will almost always be blacked-out. They generally have a mix of 6 month out discounts and 60 day-ish out.
As for getting the best rate, I think booking directly through the resort or Marriott's system will be the best. There could be Canadain agencies that have some exclusive rates, but I don'y have personal experience.
If you're staying for a week, look into renting from an owner at Harborside. Check out
https://www.familyfunatatlantis.com/ for some ballpark rates. You might be able to get a better rate by working directly with an owner on
VRBO or
TUG or
Redweek.