For best price you might be a little too early as the iPhone 14 is only available for preorder right now.
Still, knowing that, APPLE has reduced the price $100 from $699 to $599 for the 13 mini and $799 to $699 for the 13 standard size.
I dont know (dont think anyone ever knows what apple might do) but I cant see the prices dropping more than another $50. between now and the end of the year.
You can get an iPhone 12 mini for $329 after rebate from Red Pocket or you step up the memory a little and pay $379
Read some reviews on it, make sure the battery life in the 12 mini will be good enough for you, I think they improved it with the 13. Other than that not much difference but read the reviews. At one time I was really interested in the mini, to this day I liked the thought of it but bought a standard 13 because TMobile was offering a great deal for 55+ plans and pretty much we got two new iPhone 13s for trading in our older XR and 11.
Im a bit surprised what a good deal from Red Pocket. You can pick and choose even a 10 GB plan for only $30 and keep in mind Red Pocket doesnt add in taxes and fees to that $30. You can select ATT or Tmobile. the carrier that ends with the "T" is TMobile and the one with the "A" is ATT network. You can confirm with a phone call to them or "chat" I never got a phone through Red Pocket but have used their service with ATT for years, never an issue, I see you have to pay the higher price and wait for a $200 rebate after 3 months of service.
apple-iphone-12-mini
With all my rambling above, I like your idea of getting the 13 better only because better battery in the 13 and whatever other little things, the price difference not that much and since you keep your phones for a long time, MAYBE worth it but maybe not, its nice to have choices!
I dont think you will see a much better deal for a no contract phone plan. Most likely you will have to call to have it unlocked once it is paid for. (more or less read the fine print) I assume once you have fully paid the phone and got the rebate after three months you would be able to get it unlocked.