In case anyone wondered, I did end up not following through with my trade-in. (I hate it when people ask questions on forums and then just disappear; didn't want to be that way.)
People made some good points, and all the input was much appreciated. I do of course agree that my old 512GB iPhone 11 Pro doesn't have anywhere near a street value of $830, which would have been my full trade-in benefit, assuming AT&T didn't decide my particular phone didn't qualify after all. And I even agree that the next tier down, at $350, is still a good deal if you only look at it as getting $350 for a four year old phone.
But as I said, I wasn't looking at it as just that; I wasn't going to sell it privately or just stick it in a drawer if I didn't get the $830 trade-in value for it. I do have some use planned for it; a few, actually. At $830 in credits, I felt like I would have been fine giving it up and essentially being locked in for another three years with AT&T on a premium plan. At $350, I didn't see the same value in it for me personally. And I didn't want to take a chance on AT&T not giving me the full $830. I noticed that even if you go through their trade-in calculator and say that your iPhone has a broken screen and dings on the frame, it says it's still worth up to the full amount, but I was reading too many reports (and not just this year) of people saying AT&T gave them a severely reduced trade-in credit when they sent in very much non-damaged phones (I do have a couple tiny dings on my frame; no damage on any glass; phone works absolutely perfectly). Plus the AT&T policy of once you've sent it in, you can't get it back, no matter what. If I feel like gambling, I'll go to the nearest convenience store and buy a lottery ticket.
Also, as it turns out, I've been researching various MVNOs over the last week or so. Toying with the idea of paying off the new 15 PM sometime in the next few months and finding a cheaper service provider. When looking at my lock-in vs. no-lock-in options with AT&T, I also started looking at the real cost of my service as well, and wondering why I've been paying over a thousand dollars a year for a single cellular line, without any device financing included. And that's after deducting $20/mo of discounts (FAN plus autopay). Haven't made any decisions yet; still evaluating pros and cons all around.
Anyway, thanks for the responses! Gave me a lot to think about.