You are building a home network attached storage server to keep your photos, files, and backups off cloud subscriptions and under your own roof. A good setup should last you five to eight years, so you want to buy the right bays and drives the first time rather than upgrading in a year.
Prime Day price drops are the best time to grab hard drives, but watch the release cycles. When a new drive generation drops, last year's model usually hits its lowest price and gives you the exact same reliability for less money.
Remember that the 18-month window is a hard deadline, not free money. If you still have a balance left when those 18 months are up, deferred interest or standard rates will kick in on the remaining amount.