If the cost to repair is two hundred plus dollars I would sell it online as is and add some money to it when you can and grab another used device from. If you end up trying everything and just can't get it fixed. If so OnePlus has the 6t main board costing more than I paid for my mint 8gb 128gb OnePlus 6t. I'm assuming motherboard is the mainboard. What did the service center quote you price wise on motherboard replacement? I'd give that a try for sure if I were in same situation.
Then from PC terminal do: sudo fastboot -w (hit enter) and then once it wipes, push power button on your device which should still be at the screen that reads START. To do this you can plug up to your computer and open command window and manually set phone in fastboot mode where you see the 'START' screen. Have you tried formatting data and then reboot system? I know that sounds kinda dumb but if I were you I would definitely try that.
