A full-frame mirrorless camera is a long-term investment that should last you years of shooting. Buy a body that fits your current needs rather than chasing the newest specs, as the jump in quality between generations is often smaller than the marketing suggests.
Wait for the maker to drop a new flagship model. That is when their authorized refurbished stock of the previous generation usually hits the site at a deep discount, giving you a pro-level tool for a fraction of the original price.
Remember that the 18-month window is a deadline, not free money. If you have a remaining balance after that period, you will be charged high interest on the original amount, so stay disciplined with your monthly payments.