You need a phone that handles daily life without slowing down or dying by three in the afternoon. When we buy a mid-range phone, the goal is to get something that stays fast for at least four years so you are only buying once. Skipping the flagship tier saves you hundreds, and timing it right saves you even more.
Wait until a few weeks after the newest generation hits the market. That is when people rush to trade in older devices and retailers drop prices on last year's model to clear inventory. You get almost the exact same performance for a fraction of the cost.
The 18-month 0% window is a deadline, not free money. If you still have a balance left when those 18 months are up, deferred interest kicks in on the whole original amount at the regular variable rate, which ruins the discount.