This new battery however is only compatible with the Air as other phones have a bigger camera bump.
https://freshnrebel.com/magnetic-wireless-powerbank-5000-mah...
It does the MagSafe thing, but will also offer a charge through the USB-C port.
And batteries don't last forever. When you upgrade to a new phone after a few years you'd likely need a new one anyways.
Worst case scenario just sell the old one on eBay if it's still holding a good charge!
Selling a thin phone with half a battery where you have to buy the other half and keep it attached to get a proper battery runtime (turning it into a normal-sized phone) can't be the solution Apple intended. At least I hope so. And that battery doesn't fit other iPhones as the camera bump of those other phones is in the way.
https://store.storeimages.cdn-apple.com/1/as-images.apple.co...
The camera bump on other models protrudes more towards the centre of the body. And thus the battery wouldn't fit (flush) and the Qi charging wouldn't engage properly.
Seems like Apple is way ahead of you.
Other people: how dare you