A bootlooping device can be caused by many things.
Try these things (if you haven't already) to see if anything helps:
- Try a hard reset (Home & Power button for 10 secs)
- Use a genuine Apple Lightning connector
- Use a genuine Apple charging brick and let it charge overnight
- Try to connect it to different computers (mac/pc if possible) with an updated iTunes
- Put the phone in recovery mode (search DFU Mode). Careful, this can reset your data
- Use 3uTools for putting it in and out of Recovery Mode
- Use 3uTools to flash the firmware (you can't downgrade)
- Disconnect the Front Camera Ribbon from the logic board and reboot your phone
- Disconnect the screen assembly completely and see if iTunes will recognize the phone.
Otherwise, you may have a logic board issue. Did you keep track of the screws when you disassembled the phone? The screws holding the LCD bracket are of different lengths. If you put a long one where a short should go, you can cause this kind of failure. Try the steps above and get back to us with your observations.
If you are uncomfortable doing these steps, consider finding a repair shop to do the troubleshooting. A shop that also does micro-soldering can go deeper if the problem is not modular in nature.