Unable to restore iPhone via Recovery/DFU

Hello,

A family member gave me their iPhone as they want to sell it off. It had a passcode which they couldn't remember. They wanted to restore it anyway, so it didn't matter about getting back in. They had no data they wanted to extract so it was safe to factory reset the device.

I decided to use my laptop to reset the phone. I used 3uTools to flash the OS. I've used this in the past with other iPhones and haven't had any issues, so thought it'd be suitable to use.

All went well until I got to maybe 60-70% (can't remember exact percentage now). The phone was on the Apple logo and the progress bar was right near the end. Suddenly 3uTools failed and returned a "Unable to restore iDevice(-1)" error. It remained stuck on this Apple logo with the progress bar right near the end.

I forced power off and tried to flash again via Recovery mode. This time the tool fails at 11% which shows "Unable to write iBEC". So then I tried putting the phone into DFU mode. I tried flashing again and this time it just said "Unable to switch DFU mode to Recovery mode".

Thinking maybe it was my laptop, I went to my desktop PC. Same issues occurred. So I decided to try iTunes to see what that said. Unfortunately, iTunes fails as well. I keep getting "An unknown error occurred" with codes 5 or 2003. This is where I am at now.

So far I've tried 2 different USB ports, computers and 2 cables (both are original - although one is a USB-C to lightning).

Any idea what's wrong here? Did 3uTools hard brick the device (due to the 1st failed flash), is there some USB issue or is it maybe hardware (e.g. NAND)?

I'll keep trying to see if I can get it working, but so far it's not looking promising.