Need iPhone - Current Android No Longer Supported by AT&T

Need advice: Want unlocked smartphone with 4G (that is, not about to be switched off by AT&T) but otherwise comparable in size, or smaller than, my current Moto G6.
Key requirements (or is this wish list unreasonable?):

1. fit comfortably in shirt pocket -- not one of these oversize phones
2. either accept additional flash-memory chip or has lots of onboard memory
3. easy to swap over existing AT&T pre-paid SIM card from Moto G6 (I assume this is standard in any unlocked phone, correct?)
4. preferably on the DJI Fly approved list below
5. not dreadfully expensive, I guess meaning not the current model

Having so far been unable to find a satisfactory Android phone, I'm reluctantly considering an iPhone. Any help appreciated. -- jclarkw

"Requires iOS 11.0 or above.
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Need iPhone - Current Android No Longer Supported by AT&T