Erasing an iphone that power up a few seconds then dies

My ipone se g3 will power on for a few seconds then die after a battery replacement by Apple store. The guy says it has a hardware trouble. All it needed was a battery until they got it.

The guy said he'll swap it with a refurbished one at no charge, but I dont want to give it to them without first drilling a few holes in it to ruin the stored data, but then they won't take it even though I tried to reason why. I dont think Apple has an iphone warranty like computer places to keep your old hard drive.

I managed to get it powered up long enough to see it on the find-my-iphone service, then sent a command to remotely wipe it. I got an email the phone IS BEING ERASED, but not one that it finished. I turned it on a few more times the past few days until it shut off on its own in a few seconds - but not no more status.

From my understanding the phone relies on being powered up at least a few minutes to completely erase the data, and a wifi connection to email back it's done. So it's possible the data is still there and once Apple gets it may just take it apart and reuse the working parts

How do I erase the phone's data in a case like this before surrendering it to Apple?