Imo better just get a new cheap Android with an SD card slot.
Pros over a used old iPhone:
- new battery
- larger battery (most Android have at least 4000-5000mAh battery, at least more than double than any of the older iPhones)
- many recent models have Bluetooth 5 already. iPhone 7 is still only Bluetooth 4.2. Bluetooth 5 opens up many TWS buds options.
- SDcard slot, so you can copy your music from PC easily, and more capacity.
- and a headphone jack! Most cheap Android still carry a headphone jack.
Take the Moto G Power. 5000mAh battery, SD slot supporting up to 512GB card, Bluetooth 5.0, for $180 (assuming you’re in the US). You can then buy a cheap SD card to add however amount of storage you need.