AT&T is ripping me off to the tune of $1,000

I’ve been a customer of AT&T at least since early 2000 – the previous company was called Cellular One, later purchased by AT&T.
In December of 2022, I received an email from AT&T offering me a “fee upgrade” to an iPhone 14 from my iPhone 8. All I’d have to do is return it to them in the box they’d send me.
I did it online. I remember asking for a clarification of any fees attached to this “deal,” and the only one mentioned would be the activation fee.
I agreed.
The phone came, I uploaded all my data from my computer backup and sent the old phone back – via USPS, as directed – and began using my new phone.
I have automatic payments and I didn’t notice an additional fee that appeared on my bill until June of 2023 of almost $28 per month (a total of 36 payments) – which would total almost $1,000.
I immediately called AT&T customer service and the chat-person said the reason I was being charged was that they had never received the old phone. I explained that I didn’t have any receipt from the Post Office to prove I had sent it back.
The agent then promised that I would receive an email notice of the resolution of the issue within 48 hours.
I bet you can guess: I never got an email and the fees continued to be charged and paid, month after month until I finally called them again.
This time, however, the story had changed:

1. They never received the old iPhone – as before.
2. A new wrinkle was suddenly added: I’d never been eligible for the “deal” in the first place.

It seems they think/thought that I was stupid enough to willingly pay $1,000 for a phone from them that I could have gotten from any other provider for free.
Today, I went to my local AT&T Store and talked with the manager. He was very kind but the best he could offer was I could trade in my iPhone 14 Pro, get ~ $350 for it, but still be on the hook for the rest of the $700 I still “owed.”
As a last resort, he suggested I call the “Loyalty Dep’t.” of AT&T.
I called this farcically named “Loyalty Dep’t” to see if there was some way to resolve my problem.
Can you guess the “deal” I was offered? Zip, Nada, or, in effect, “We got you, sucker, and we’re not letting you off the hook.”
The agent said she would shoot it up the line but that I shouldn’t expect much.
What will I do?
Part of it is what you’re reading now: a warning against AT&T.
Several friends of mine have been teasing me that I was being a fool for staying with AT&T out of silly “loyalty” – to the tune of $190/month for 2 iPhones and an old flip-phone for my brother who refuses to upgrade, including $72/month for 6 gigs of internet access – and I should do what they did: go to T-Mobile for a fraction of what I’ve been paying.
BTW: This included a ~$20 discount for “customer loyalty.”
When I finally get out from under this AT&T ripoff, that’s exactly what I will do.