I know you said you tried all of the boot-loop suggestions but you don't tell us which ones. So for the record, I'll post some ideas below and tell us if there's anything in there that you haven't tried. That way we'll be able to help a you a bit more.
A few things to consider:
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)
- Check the condition of all the flexes (with magnification)
- 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 probably have a logic board issue. If you are uncomfortable doing these steps, consider finding a repair shop that does micro-soldering; they can go deeper if the problem is not modular in nature.