I read an article a few years ago about smartphone trade-in options and the one that generated the most money was a private party sale (just like used cars). That means Craigslist or eBay. That article also reviewed the carriers (bad value) as well as some third-party resale companies.
Of those the one that offered the best value was Amazon, but they offered store credit rather than cash back. If you want cash, the service that returned the most money was Gazelle.
There may be new players (does Apple offer trade-ins?) these days, but for sure, the third-party services are competitive and relatively hassle-free, important for those who don't want to deal with a private party sale via eBay, Craigslist, whatever.
That may factor into your decision about whether or not you want squeeze every last dollar from your old phone or if you're willing to give up a few bucks for the sake of convenience. Since you neglected to mention this in your post, it's difficult to make a specific recommendation to you.
I have used Gazelle once and Amazon once; I can't be bothered with a private party sale. I will probably continue to use Amazon as long as I remain an Amazon customer since I'm going to spend money there, I might as well claim the extra twenty or so bucks that Amazon will offer versus the other cash-back companies.
Good luck.