iPhone boot loop after screen replacement

Hi.

I bought a broken iPhone 6 back in January. The guy who owned it said it had a broken LCD after it fell on the floor, it hit the left chassis and the glass near the front camera was chipped. The LCD was then showing some white stripes, and he tried to update the iOS to solve the problem and it was on a reboot loop. He said the update was successful but it failed to start for the first configuration. It was also supposed to be working fine before it was dropped. He had replaced the battery but it was still stuck in a boot loop but didn't replace the screen. The phone has not come in contact with water.

When it arrived, it had no charge. I put it on charge and it booted to the welcome screen. The lines went past the middle of the screen and the touch didn't work from the lines till the right side of the screen. I re-seated the display cables and the touch only wasn't working on the lines. The phone seemed fully functional apart from that.

I left it for a few months and last week I changed the screen. Before placing the brackets I tested the screen and it worked fine. So I put the brackets on and booted it up again. It was in a boot loop :/ The Apple logo would come up and then it would restart and the lower part of the housing was getting warm. It did this until it ran out of charge. I charged it and after hard resetting it did turn on but turned off after a bit. The battery was 10-15%.

Thinking the battery wasn't calibrated because of a few months of no use, I let it discharge, charged it while off for 5 hrs. When I turned it on it was in a boot loop and I left it to discharge. It has turned on a couple times but not for very long and goes back to a boot loop.

Yesterday I put it in DFU mode and restored it but it gave me error 9 and the progress bar stuck halfway. I tried rebooting but it was stuck on recovery mode. Today, fearing long screw damage, I checked the bracket screws but they were all of correct length. I then disconnected the display and tried to restore without display. It restored successfully, I disconnected the battery and reconnected the display cables. It was stuck on the Apple logo and boot looped. A couple of times after hard resetting a progress bar came up under the Apple logo only momentarily. It has now ran out of charge.

Sorry for the long post, wanted to get as much detail in there as possible.

Any ideas what the problem would be?

Thanks