Here's the thing though: don't invest in iPhones. Just jump on your carrier's yearly upgrade program or better yet the Apple upgrade program and never feel remorse again. It's not worth the hassle anymore to deal with selling an iPhone amidst the sketchy and absolutely flooded market that is secondhand smartphones.
Over the course of 12 months, I pay a total of around $650 for a 256GB iPhone X with the $199 AppleCare+ included, and then as soon as the next generation is on sale I get to swap it out without having to deal with buyer's remorse on the next model, the horrible secondhand iPhone sale process to get rid of my existing device, etc. That feels like a great deal to me - the total out the door would have been double that, and if I was trying to do the yearly upgrade on my own by selling the phone I'd probably make what, $700 of of it? I'd be coming in right around the same dollars put in but with lots of extra work each year.
This way you're excited about what next year brings and not disappointed!